Depending on where you live, that's not accurate. T-Mobile is perfectly adequate (and sometimes even better) compared to AT&T and Verizon, except in more rural and isolated places.
I'm in a fairly rural area, and use a T-mobile MVNO (ting). In this area, there are holes, but they're the same holes that ATTVZN have.
I don't live in a rural area (~1000 people/sq. mile), 40 miles away from NYC. At least as of a year ago, T-Mobile's coverage map of my area was "occasional service outdoors".