^^ I guess it all comes down to what one finds exciting or musical.
For example, I enjoyed the IE8, it was 'fun' and I prefer it over the SE535 & SM3, SE530 & CK10 and maybe W3 (been too long to remember exactly). But, at the time I had the IE8s I missed the clarity AND instrument separation the UM3X gave me. To me, such detail is part of the massive enjoyment of and connection with music. It's almost like preferring stereo over mono recordings, though I know there may be those who prefer the latter.
For me, the fact the UM3X could place & define instruments so well made it render music more faithfully, more lifelike. And this kind of excellent instrument separation and placement does not always cause me to be analysing each and every instrument in a given piece of music. At times I do, but out of sheer enjoyment or wonderment at the way a particular instrument is clearly defined when, for instance, hearing a good bass & drums combo or a string quartet. This sort of '3D' (for lack of a better term) type of presentation is tremendously exciting. I don't find myself being distracted by this, quite the contrary, it becomes very engaging, where I find myself often listening for longer periods than I really should.
With my ES3X this happens to a greater extent as the clarity, imaging & excitement are even bigger.
For me things become 'problematic', and in fact they have become 'problematic', when a certain frequency is very dominant or lacking, something I recently experienced with the SE535 & SM3, with the latter being quite veiled & muddy, and the former being a bit bright and bass light. To me the better the instrument separation & imaging the bigger an asset an IEM has and therefore the more enjoyment I get; if you then add balance & neutrality, well...that's what I get from the UM3X, CK100 & ES3X. The CK10, W3 & SE530 also had, to these ears, some kind of imbalance that made me get rid of them.
But again, there seem to be plenty of SE535 enthusiasts these days - though I attribute this more to the new design & aesthetics of the SE535, ie "This IEM looks and feels so, so good and is so expensive that I've got to like it somehow" plus the fact that it's one of the latest new & shiny toys out there. The SM3 is an IEM where quite a few people have tried to change its original sound sig either by trying all manner of tips, or tried modding or EQ'ing them, which makes me think quite a few of these people are really trying to convince themselves the SM3 really is a good IEM when it isn't.
(The above paragraph will no doubt provoke reactions)
Of course there will be genuine people who will prefer the SE535 & SM3's SQ over the UM3X's. My ears led me to believe that if a blind test was carried out, more people would actually prefer the UM3X. My ears also tell me that in a blind test more people would prefer the UM3X over the CK100, but today I still find the CK100 just a little closer to MY preference, and can't wait to A/B them again against my UM3X.