Quality IEM makes all the difference to me. Also depends on the headphone amp being used. For example I have used an Aviom and a P16 and with the same pair of IEM I prefer the sound of the P16. It is brighter sounding than my tone sounds through FOH but I can live with it
I use $1000 6 driver custom molds for IEMs with a top of the line Sennheiser unit. I've never had luck with IEMs whether they're custom, expensive, 2 driver, 3 driver, shure system, etc....guitar always sounds bad no matter what we try. Everything else sounds good. The guitar sounds great out front though. It's just the fact I don't like a guitar tone pressed against my ear drums. It's like sticking your head in front of a guitar cab speaker and hearing that. Won't sound good.