I've never tried anything higher than dual driver, but I do know this. The crossovers in IEMs are R/C networks - the most simple form of crossover circuit possible.
And with a simple crossover like that, you get all kinds of phase shift, and harmonics and so on and on.
So IMHO, I think for simple IEM's with the most consise sound, I would prefer a two way, (single crossover) that way, there's less to go wrong.
The future for complex multidriver IEM's is with external digital crossovers.
There's different types of crossovers in custom IEM's I believe, for example the TS842 uses "a hard crossover of the 2nd order with 12dB step increments for a more natural sound", whatever that means.
As far as disharmonics and phase shift goes, I haven't been listening for that in particular with multi-BA stuff, but definitely haven't picked on any of it either.
The weakness of the ER-4 is it's a single driver unit so the imaging is far too concentrated to be able to layer and dissect the details, and the 2-XS (well, K2 SP, exact same unit...) is dull, bright, lacking high-end extension and just plain jane boring.
Not sure about the UERM.