You won't get fatigue from UERM. Typical fatigue is caused by harsh and overbearing treble, which isn't what UERM is all about. In fact it has one of the best treble presentation - crystal clear and airy while smooth. I considered it a strength. Comparatively my jh13 could fatigue me easier with the somewhat spiky treble, even thought it is warmer and bassier than UERM.
On the other hand, the lack of a fluid and impractical bass could impede your enjoyment for genres that call for more bass. Personally I highly regard V6S as an all-arounded, reference quality and thoroughly enjoyable ciem with enough transparency to be considered totl sound but with good bass at a very reasonable price.