Xfinity Mobile looks real good on paper...
But judging by their own site,
https://forums.xfinity.com/t5/Xfinity-Mobile/bd-p/XM, feels like a bunch of amateurs running the place. I say if u want to try them out, fine, good ahead. And by try-out I mean, use an old compatible BYOD iPhone and cancel if ur not happy within 30 days, then u only lost a few bucks.
If u dive head-long into it, buy hundred$ of dollars of phones from them, expect them to make good on debit card, port your numbers with dispatch and efficiency, expect quick reply and problems resolution, better spend some time on that forum first.