I definitely have "serious" questions concerning Fi (Fi is an interesting animal) but it's been my experience that Fi'ers really don't want/like those serious questions - and sometimes the dialogue becomes delicate, stressful, or even heated (both IRL and on the forum). So, if my question fits the "uncomfortable" category, just politely let me know and I'll skip on my merry way. I come not to stir up the pot, but to understand the animal that is Fi.
I'll ask it bluntly: Why do you guys not clearly express what Fi is feeling/thinking? If you're upset, mad, angry, disappointed, frustrated, feeling ignored/betrayed, etc. - why not just tell the other person exactlly what those feelings are, which then gives that other person something tangible to work with. Basically, my confusion with this part of Fi is that if you never voice what's going on inside (plainly, clearly), then you'll probably never get any results that resemble what you're hoping for/wanting.
I express my Feelings a lot, as far as my judgments go. I hold back if my Feeling is so, well, critical that it would be of no benefit to anyone.
If I don't have much of an opinion, it is because I don't care. If I just go with the flow, it's because I don't care. If I see the movie you want to see nearly everytime, it's because I don't care about any other movies. When a movie comes along that I want to see, then I will make it known & see it.
Another thing is, the Fi mindset does not like to affect other people nor be affected. We kind of want to do our own thing & let others do their own thing. We don't want to make other people responsible for our feelings - feelings meaning both reasoned out conclusions & emotions. This means we do not push for our way a lot with other people. We'll pursue what we want on our own time, which is likely why many of us like a lot of space in relationships. In general, we often don't care about a lot of stuff other people care about & we care a lot about stuff other people don't even think about. This means we're not giving up our preference so much as no having one in those situations, but when we do have a preference, we will ask you to take note.
Now as for emotional feelings, I do not express those for some of the reason stated above - I don't feel the need to influence others with my emotions. Emotions communicate significance to others, and Fi-doms tend to see valuation as an individual process (whereas for Fe, it's very much a consensus process). Emotions are then internalized and processed as a part of an ideal forming process. This is only unsuccessful when the other person really is doing harm & needs to be told to stop - unfortunately, many Fi-dom just blow up at that point. The Ne aspect allows one to creatively dodge confrontation also. Much of the time, stuff just doesn't seem worth the energy....
If we confront in a healthy manner, we often seek a win-win, because the idea of implying our values are more important is yucky. It's too easy to see the tables flipped; we'd never want someone to do that to us. I suppose the win-win approach seems passive or maybe compromising in negative way to non-NFs, but I think it's a very positive style. I've never had a real problem asserting my preferences when necessary, so I can't say I relate to the very passive INFP stereotype. I'm known amongst family/friends for being pretty blunt & only accommodating when it's not a major sacrifice to my own needs (and I mean valid needs here, not sheer selfishness). I also don't want to make others obligated to my individual preferences either though.
As for emotions, when Fi-dom express emotions, they may do so without much show, which leads others to not take it seriously. People frequently question the depth & authenticity of my emotions because I am not prone to dramatic display. Fi-dom are a LOT less expressive than people online in these MBTI communities seem to realize.... Anyhow, when you have your emotions invalidated, you tend to not bother communicating them.
Which brings me to the next point....Fi feelings, both the feeling-thoughts & emotions, are often out of sync with those around them.
Expressing all of your feelings candidly can mean:
1. you offend, shock, and/or alienate people; they cannot grasp nor accept what you are telling them because no one else feels that way...it's too abstract/weird/foreign for them
2. your feelings are judged as wrong, strange, or invalid, and by extension, so are you
3. your feelings are so conceptual in nature that explaining them in everyday terms is hard without losing meaning, and you wind up being misinterpreted
4. you spend a lot of energy explaining something quite simple in your mind, only to result in one of the above, and so you just give up
I can't tell you how often my feelings are questioned, invalidated or judged negatively because they are not expressed in the way others do, are not what others often feel in such situations, or they are hard to express accurately, so nuances are lost, and I wind up misunderstood. Not to say no one every understands me or does not make the effort, but the back & forth to reach an understanding is tiring. Again, it just doesn't feel worth the effort sometimes.
Also, Fi asks for an appropriate expression for a feeling, which is hard to find. The depth & nuance of Fi feelings go beyond the neat little list of adjectives most use to describe feelings. Those words feel cheap, shallow, trite, phony, and generally inadequate to express the feeling in full. Other means are often used, which allow for a greater depth of expression (ie. art, poetry, music, philosophy, etc). Those outlets allow an intensity that everyday communication doesn't because it's not within the frame of what is socially acceptable or "regular".
If I could draw an analogy, it'd be something like this: The Fi'er
really, really wants a shiny red sports car for their birthday (or a penguin, or a pony, or something along these lines). When someone asks you, "What would you like for your birthday?" (cuz they want to get you something you'll really like), you just say, "Ehhh, I dunno, a gift card"

And, so, you end up getting a gift card and you feel upset or kind of sad that you didn't get the sports car. Well, you didn't get it cuz you didn't ask.
Or, the Fi'er wants to spend time with someone they love. And they talk to the person they love on the phone and the person says, "Whatcha doing tonight?" And the Fi'er says, "Heh, not much."

So, the other person thinks, "Hmmm, OK, they aren't doing much and I'd like to see them, but I know they like their space and I don't want to impose, so I'll just leave it at that and maybe see them next week sometime." Then they say, "OK Fi'er, good talking to you, we'll catch up next week." And then find out later that Fi'er was very upset and wanted them to come over.
Why doesn't Fi'er just say, "Yo, check it out. I want a red sports car for my birthday and I also want you to come over tonight and hang out with me cuz I miss you." Straight up. Tell it like it is. Done! Finished! Wish granted. It can be so easy, so simple. But, instead, you never voice these Fi desires and so the other person..........never knows what it is you wanted. Inevitably, this will cause feelings of hurt and neglect and "he just doesn't know me".
I've heard 2 different reasons for this "holding back": 1) It's hard and/or uncomfortable to express. 2) Fi'er doesn't want the other person to do something for them "just because they told them they wanted it" (thereby "prompting" the action); instead, Fi'er only wants things to be done for them without prompting, completely of the other person's free will.
If I compare Fi to my own Ti, then yeah, I can relate that it's sometimes hard to express "complex thoughts" into mere words (and some things get lost in translation), but if I really want something, you better believe I'm coming out with it eventually.
Why Fi'ers? Why?
What you describe as preferable is a directive communication style....an INFJ might say that, but an INFP will use an informative style. It's not that an INFP won't tell you what they want, but the prefer to express a need/want in general & let you figure out how to accomplish it; or they'll ask in question form, so you have freedom of choice. Sometimes it annoys me when my INFJ bf tells me HOW to do something; it seems bossy and condescending. People tend to say things they way they want to be dealt with, and INFPs like freedom to do it their own way, so they give that to others.
I'd say, "I love red sports cars & would love to get one for a gift someday" and "Not much; would you like to come over tonight & hang out?".
If I don't ask for what I want, it's likely because it is too much to ask for (I mean, who really is going to buy a sports car for a gift except the very wealthy?), but then I also don't expect it. Even though I admit I dream big, I don't want to seem, well, high-maintenance or unreasonable, nor do I want a sense of being in debt to someone because they went all out for me. I'd rather buy that sports car (or whatever) for myself; it would mean much more to me. I learned a long time ago that most people don't give without expectation, and I want to be free of that, and I don't want to impose any expectations on others. I don't ask much of people so that I am not obligated to them..... When I was a kid, it did help that my grandparents would give a us a "budget" for gifts. They'd say we could pick anything out up to $200 or something. I had no problem naming stuff then.
I also don't expect nor want people to read my mind. I can't relate to that...
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And I am apparently not an INFP of the playful Ne banter variety....
/Fi wall of text