I think one of the key components of an undershirt is the collar.  In this case, this collar is thin, it stretches out, and it sags.  When you're wearing a shirt over it and all people see is the little white V area, but see a sagging stretched out collar, it looks terrible.

I got a large, which fits true to size, but the spandex in it makes it tight on the body, and I wore it in 110 degree heat as an undershirt and I was cooking it in compared to the 100% cotton shirts I've also worn in the same climate.