Any Downside to Getting Phones from T-Mobile?

[Background: I've been with T-Mobile since maybe 2013. I now have 10 lines on a Magenta plan. I've never purchased a phone from T-Mobile; I always buy unlocked from Google or BestBuy or used.]

Is there any downside to taking advantage of the offers that result in bill credits? I have been leery of bill credits since I'm currently on an amazing plan ($105 for those 10 lines), and I really don't want to mess that up. On this subreddit I've seen people complain of bill credits ending prematurely (although not recently) and I just don't want to deal with the headache because overseas T-Mobile support will literally tell you anything to get you off the phone/chat and they never actually do what they say they will.

I understand the best promos aren't available on Magenta but that's no big deal.