The only pro to an T-Mobile branded device is thier specific features like wifi-calling and voice over LTE "VOLTE" are built in these device already.. Plus you get the benefit of having pre-installed apps on your device as well..
However when purchasing a unlocked device, you are not guaranteed that it will work on the network because unlock devices tend to have international bands and not the right ones for the US.
It is not a bad idea to purchase a unlocked device because you have the freedom to jump carriers and have less bloatware "pre-installed apps" on the device..
Just make sure you get the right band phones preferably the ones made to work in the United States..
Brands like HTC, Google, Huawei and Blu have unlock phones available for the US that will work with T-Mobile perfectly.. Wi-Fi calling and VOLTE work on these branded phones so you get to take full advantage of the network!
I have T-Mobile and currently use the Google Pixel unlocked directly from Google.. I have no network issues with my pixel!