In ear monitors and mixing question

I've never tried anything that high end, just the generic (not custom-molded) IEMs that sell for < $300. I've gone through three pairs over the years (they break easily).
 
I like them a lot as an alternate,  rather than a primary, reference. But that's because they sound so different from my speakers. Different enough to sometimes identify weaknesses in a mix, hence their value as a secondary sanity check.
 
However, my experience was that IEMs suffer the same problem as most headphones: their nonlinear compression tends to make everything sound better, making them an untrustworthy standard. A mix that sounded just fine on the IEMs would often not translate well to other speakers, or even to headphones. By contrast, if it sounds good on my speakers I can be confident that it'll sound good anywhere.