Hey, you know I'm not a 'fundamentalist' when it comes to any IEM, but I came to appreciate my UM3X quite a bit when I had them and all the more now after my last two recent purchases, the SE535 & SM3.
Here's my take, anyway: To me the UM3X is a very balanced IEM, quite neutral and yet very exciting to listen to. I wouldn't say the same about the W3, though I do think it's a great IEM, just not as refined.
When I first had the W3 I was quite wowed, very impressed after coming from the SE530 - an IEM I disliked a lot and not just its SQ - but afterwards I found the former fatiguing, specially after an hour or so when playing music moderately loud, bass and treble on the W3 seemed just a bit too much for my liking.
The UM3X did not wow me at first, no flashy presentation, but after 20 mins or so I began to be wowed or rather drawn to the sound in a more gradual, subtle way. And almost without noticing I ended up being unable to put them down for about 6 hours straight, a first for me. I experienced absolutely no fatigue and I could play music more loudly quite easily if I wanted to.
Instrument separation was stunning. The UM3X is not bass light at all, and that's a well known fact; it has more than enough bass, and covers the low frequencies more convincingly than the W3 & IE8, in my view, it goes quite low remaining detailed and having plenty of impact. Treble is less prominent than on the W3s but not recessed, but sort of 'kind'/ 'polite' on the ears, and also with plenty of detail. Mids are to me the best I've heard to date, full stop, save for my ES3Xs.