So I have sold my UM Pro 50s. So while I no longer have them, I have always taken care of my stuff and enjoyed my time immensely with them. I never had a large roster of IEMs I have owned but I have always owned some expensive stuff for my age. I started with my Monster Turbine Gold ($350 or something) back those 8 or so years ago. After they finally broke apart, I purchased the UM Pro 50s. They not only were a different quality of product all together, the sound was amazing. It was what got me into the BA-IEM game instead of Dynamic.
I deeply enjoy the sound and quality that it produced. While I have not had the problem with the MMCX connector, I feel that its a large problem with many IEMs that choose to use that type of connector. Unless if the manufacture process improved, Westone will continue to face problems. The housing that holds the MMCX in the IEMs is decent but not encasing enough. Several flaws is the way the connector itself is shaped. Since the female connector (IEM side) is actually round with two small long pins, there's not much to grip to. Compare that to the 2-pin solution which has a shape that allows from gripping by the two shells.
There are few ways to solve this problem, from the way I see it: Use a special adhesive that bonds the metal connector to the 2 plastic shells or produce a partial redesign on the outer portion of the MMCX connector to make it less "round". Seeing as I am not versed at the guys who actually work and produce these things, I'm merely stating based by what I am seeing.