I think with power-banks, just make sure you get a good quality one from a reputable brand, and make sure it actually has enough capacity to charge your phone when required. Samsung is certainly reputable, but you need to make sure it's not a knock-off. Where are you buying it?

FWIW I've been seeing a lot more fake Samsung in the last few weeks, including S8s.

 

I've read good things about Anker power-banks here on AF.