just like any other speaker, IEM's aren't automatically going to sound perfect with audio running through them. many of them these days have way too much bass, which could be the direct reason for the issues in this thread.
IEM's should be EQ'd, just like any other speaker used at a venue. this isn't always possible depending on the gear, but if it isn't EQ'd, then if it's too bassy, the audio will be too bassy.
i find even with "flat" IEMs, i have to cut bass frequencies to allow "room" in my ears for the other frequencies to come through.
i typed more detail here:
https://forum.fractalaudio.com/threads/dialing-tones-for-use-with-iems.145667/post-1722416
but here's an example of my EQ for a Shure SE215
without that bass cut, everything sounds muddy and i can't discern anything. but it still sounds good and full to me, but not like a mixed album or anything because i'm not listening to a mixed album.