Really (more about the INFJ being the least satisfied, I hear about the INTP thing all the time)? The only reason I wonder is because I'm usually hearing from INTPs about how happy they are in their relationship with INFJs, and how long the relationships last, versus what happens with other types. Maybe they are happy while the INFJ isn't so much, but that didn't seem like the case. I should find this out. I keep seeking out INFJs and it's been hit or miss. I've even gone as far as scraping OkCupid (dating site) to see which types I match best with on average, and the number one spot is pretty much a tie between INFJ and ENFP and after them there is a huge gap and then the rest of the types. I'm glad NFs are overrepresented in the female population on OkCupid, because this ups my odds of finding one I'm compatible with. The data did kinda throw me, I guess, because while there tends to be an instant connection between myself and ENFP women, it usually turns sour at some point (they start projecting and showing just how much attention/support they need to be happy). I guess what I'm saying is that I hope the same doesn't happen with INFJs.
Actually, I have a sister that's INFJ, so I have some sort of lifelong experience with them that should help. She always prefered it when I was in kind of a weaker position (victimized, etc), and I'm not sure I like that, but otherwise everything was ok. This is a big deal given the fact that I asked her to register on OkCupid and answer the questions so I could run my script, and our match % and friend % ended up in the 60's (not good). So if things were this decent with someone having such a low match, then I'm hopeful for what will happen when the match % goes up into the 80's and 90's. Then again, maybe my sister has a lot of annoyances/problems with me that she's just bottles up. She does that kinda stuff. I would ask an INFJ-INTP couple to register on the site to see their match and how well it relates to their experience, but I'm too afraid it will ruin their relationship.
I heard of another study where NFs are usually the most satisfied with other NFs, followed by NTs, and SPs are usually the happiest with NTs (even though NTs don't like the SPs as much), but that wasn't very useful because it didn't break it down on a type-to-type basis.
Also, I wonder about that study you mention. What type were the unhappy INFJs with, and what types were the poor-performing INTPs with. The INTP-ISFJ thing happens often, so I wouldn't be surprised if it's them complaining, but I'm sure the INTP isn't exactly rico suave anyway. Also, the INTP-INFJ pairing seems like the happiest for INTPs (which is what I was looking at), not necessarily the most common. But then again, if the INFJ isn't that damn happy, then what's the point.