Perhaps someone more knowledgeable can follow up here, but is it possible that a given phone that worked well with the underlying TMO network might not work as well on the new AT&T network, not because of coverage, but because that phone isn’t optimized for the bands involved/required, for example in this 4G LTE list:
AT&T – 2,4,5,12,14,17,29,30,66
T-MOBILE – 2,4,12,66,71
In other words a phone that doesn’t have bands 14, 17, 29, and 30 would necessarily be deficient, and lead one to (incorrectly) believe that the AT&T network is inferior.
As an example of a lack-of-bands phone (for both networks) is the Moto G100, and even though Motorola is selling it in the US, from what I can tell, based on their specifications, this is not a phone anyone in the US would want.
Edit: To be fair to this analysis, one coverage issue that involves bands (again please someone correct me if I’m wrong) is that Band 71, used by TMO (exclusively), running at a much lower frequency (600 MHz), has the capability of reaching further into remote areas because the lower frequency travels further, leading one to (correctly) assess that the old RW <4.0 TMO based plans have better coverage because it used Band 71 and RW 5.0 doesn’t.