I understand the thing about wanting certainty. That does make sense to me, although personally I don't care about having an opinion about everything (I don't have much of an opinion if I don't care). But I wasn't trying to say that it was about dishonesty for any type. I also understand things like changing your mind or your perspective if you get new information. That can be necessary and right. What I find very unsettling is when someone states something as absolute truth in one moment - and I'm sure that in that moment it IS absolute truth to them - but then a couple of days later, that "absolute truth" has changed to a totally different and even opposite opinion. I am more used to this (to a certain extent) with a type like ENFP. There's more of a tendency to explore different options, to say things as a way of trying them on for size, but it can change very quickly. To someone like me, anyway. I don't know if an ESTJ is likely to say one thing as their absolute truth one day, and then say something completely different a few days later. With the Ne-Fi it seems like the "truth" is based around the feeling in the moment. THat's probably an oversimplifcation and I'm probably going to annoy any xNFPs who read this. But that's how it comes across.
I know that xNFPs do tend to be a whole lot flakier than xSTJs, in a lot of ways -- so I'm thinking maybe a lot of the difference is that the xSTJ isn't as impulsive and in the moment and therefore has fewer times that they'll want to change paths? Also, I think it comes from Si being a much more exact function than Ne. Even if an xSTJ were to make exactly the same statements as an xNFP and then change their minds later, it might seem less like they were trying it on for size because they'd make it very obvious that they fit the new opinion into the old framework? I dunno -- appropriately enough, I'm thinking out loud here.
I have an xNFP friend who repeatedly gets involved with guys from the same culture, usually in the most precipitate and dramatic fashion imaginable. She's sworn several times that she'd never take this approach again, or that she'd never love again, or that the one she was currently with was IT. A few months later, literally, she'd be involved with another guy in an almost identical way, again. End result: I tune out much of what she says and just wait to see what happens next. I ended up in a couple of situations surrounding this where I felt somewhat used and betrayed (ie. she asked me for advice and even financial help but didn't tell me what was really going on.) So I don't know how much choice I have.
I think I see the difference that you're talking about now, in more detail.

There's a lot of losing touch with Si, going on in that behavior. A lot of being in the moment and forgetting past experience -- which is something that xSTJs are rarely guilty of!
Sorry, I'm not talking about ESTJs at this point, but it's just to illustrate why I handle the "constantly changing truth" thing badly. You may believe something fully in the moment, but if you don't stick with most of those things for more than five minutes, how do I know when to trust what you say, except to ignore what you say and watch what you do?
I see what you're getting at. I have an ENxP friend and an INFP friend who are both similar to the example you gave. The INFP will drive me nuts from acting like that -- I actually vented about her earlier in the thread, somewhere -- but I'm used to the ENxP since I've been her friend for so long.
It's interesting -- I hadn't thought of these differences before, because I had only thought of the similarities. Yes, xSTJ opinions are also in the moment, and yes, they see those opinions as being absolute fact, but they're also much, much better about giving logical background on those opinions, even if that logical background is "I read it in the National Enquirer" -- whereas an xNFP might go "I just know that it's true, okay???" or otherwise take it personally. Which is one reason why I tend to get into arguments with xNFPs all the time -- because when they have patterns of behavior like the one you described, I may end up getting frustrated and confronting them about it. Not in a hostile way, but reminding them of their history in behaving that way, how it's never worked and why keep trying. It doesn't always go well, and when it does fail, I end up resorting to exactly what you've been doing and distancing myself from them, watching and seeing what happens, with the difference being that as I'm watching and waiting, they know exactly where I stand on their behavior, so they may discuss the issue with me in more depth after the behavior fails for the umpteenth time and they start to see that I'm right.
I'm far from perfect, I'm not trying to set myself up as such. But in such a case, say if I'd had my heart broken - in the moment it might feel like "I will never ever get over this, I can never love again." However, I'm not going to go around saying it. Because I know that opinion is probably going to change once I eventually move past the hurt. At the very least I don't want to look like an idiot. I'll keep that aspect to myself and work through it and then see if I feel like loving again.
This does sound like an introvert thing.

Or even just a personal thing. I have INFJ friends who LOVE to vent exactly like that when bad things happen. (Not necessarily venting to everyone, but still venting like that). And that might be another way that xSTJs are more palatable to you -- we hide those feelings most of the time! If I feel like I can't ever love again, I may tell you once, in private, and may cry a little, but that'll be it.
I have a hard time understanding why people - just PEOPLE, not types - can't acknowledge that their opinion is currently in an evolving state, or that they don't know about their subject, or that they might change their minds. Instead of saying "this is how it is. This is absolutely how I feel and have always felt" and then a week later they say "this is how it is. This is absolutely how I feel and have always felt" - but, um, what they're saying is totally different from a week earlier. Is there a feeling that you're showing weakness by showing any uncertainty or indefiniteness?
I feel like the bolded is an incorrect assumption? Unless you've actually heard people say that, in which case I take it back. But I don't think any xNFP or xSTJ would presume that about their feelings about a topic -- because they're very aware of when they've replaced one opinion with another.
Yeah, it's true. I do worry about people not being what they seem. I think people very often represent themselves as something that they're not, for their own selfish ends. It's kind of a complex I have at this point. Maybe I've witnessed it too much. I haven't experienced hideous betrayals personally, but I have certainly felt betrayed and deeply wounded and I have seen people undergo much worse betrayals than anything I've experienced. I don't really believe that there is more than a very small number of people worth fully trusting, I'm afraid. Perhaps it's the type 6 as well. However, INFJs generally seem to have issues with this sort of thing - coming to feel that they can't trust people who always seem to be changing the goalposts, for instance. We don't care for what comes across as unreliability, even if it isn't intended as such.
I wonder if this is why I'm friends with so many INFJs? One of my theories of what makes for lasting friendships is that the best sets of friends soothe each other's neuroses. And in a perfect situation, INFJs and ESTJs balance each other out in just the right way, in that regard.
Either way -- I understand that worry, especially if you've had sad experiences with that.

Nonetheless, I would say that everything you've been seeing with xNFPs is nothing more than the emotional equivalent of flakiness. They aren't actively something different from what you see -- all their certainty is still their certainty -- but just like any other flaky person of any other type, you can't take them at their word that they'll do a particular thing at a particular time. So as long as you expect flakiness, your expectations are met.

Like how I expect my INTP dad to be 10 minutes late to everything. Same concept? I know that's probably not what you want to hear

but if trust is a problem, then the problem may be solved by trusting them to do what you have seen them do historically.
I agree those that xSTJs seem like particularly reliable types for the most part. I probably have some issues with how extroverts generally express themselves. I think regardless of type other than extroversion, extroverts are more likely to just throw something out there to see how it sounds/feels. But because xSTJs do seem reliable, partly for that it surprised me to learn that they may sound definite about things for the sake of sounding definite (if that is indeed how it is), and not because they ARE definite about them.

True. We often make ourselves definite about things -- like repeating a mantra.
More thinking out loud: I think another notable difference might be that the sorts of topics that xNFPs think out loud about might be the sorts of things that xSTJs don't. For example, the situation I vented about before, regarding my INFP friend, irritated me because she said she'd do something
on a moral basis and then rationalized herself out of it and didn't think it was a big deal. The flakiness I could handle, but the backing out of her
values I just couldn't figure out. xSTJs, I think, can backpedal on most opinions, but value-based and emotional statements are where we draw the line. So, you wouldn't be as likely to be in a situation where an xSTJ was an emotional burden on you, like the xNFP was.