Really, I think everything has its share of compromises, be it headphones, IEMs, or speakers. Some people can't stand the feeling of stuff shoved into their ears, while others can't get a good fit with IEMs for whatever reason. Also a lot of IEMs create a vacuum-type sensation that can be very distracting. Most IEMs can't replicate the spaciousness of fullsized headphones, and for some people the overall size of the soundstage is really important, even if it means instruments sound like they're clustered on either side. I'm not one of those people however. Cohesiveness of the soundstage and the relative positions of one instrument to another are more important to me.
I'd also argue for another parallel between IEMs and speakers: physicality. With speakers, your body feels the sound. Even the bassiest headphones lack that bodily impact that speaker bass has (with the exception of the H2+ perhaps, but again, that's basically a pair of little speakers dangling on the sides of your head). IEMs have that bodily impact, but on a much smaller scale. Stuff like FAD's BAM can actually be felt... you can feel the air being moved.
People have the misconception that IEMs are going to be inferior to fullsized headphones, that for the same amount of money you get more with headphones when the opposite is actually true (you can buy more IEM for your money). I guess in part it's a psychological thing, because IEMs are smaller, and bigger is often seen as better? Plus there's the idea of compromise. Obviously if it's portable and small, there has to be a catch or something. Also it wasn't long ago that many IEMs did suck, and your money didn't go very far. Things have most definitely changed however. I think IEMs are one of the areas in this hobby where innovation is strongest, where there's genuine improvement in products from one year to the next.
I agree. It's not always about the size. It's the motion of the ocean lol ;)
One reason why I left full size cans and much prefer high end IEMs.