T-Mobile Galaxy S8 unlocking question (using Verizon or AT&T SIM)

Hi there! I've got a T-Mobile Galaxy S8 which is not eligible to unlock yet (balance hasn't been paid off.) For those of you with fully paid and unlocked T-Mobile sourced S8s, I was curious if you've put a Verizon or AT&T SIM in it and how it works (meaning any drawbacks?) Bonus points if you have a legacy unlimited data plan on one of those carriers and curious if the Mobile Hotspot feature works. I know it works on the fully unlocked variant of the S8. Thanks in advance!