I own a few of the Stanley fat max pliers and I like them fine. Some people will say only Knipex or NWS or Snap On etc will do and blah blah blah... sure, those brands are all very very nice tools.... but, nothing wrong with the fat max, especially if it’s all you’ve got and that’s your budget....
honestly, if you own all of those already and like them then It doesn’t matter if anyone else likes them! I think a lot of guys can sometimes feel as though they need someone else to confirm for them that a low cost tool that’s not “snap on”, or another brand with a huge fan base on here, is good enough for THEM .... if it works for you, who cares what anyone else thinks.... however, I do see the point in your question as to whether or not they will last, and I’m my experience, having used mine for about 2 years regularly, they seem the same as the day I bought them....
If it was a bad tool, you’d be able to tell right away... I recently bought a cheap set of Kobalt pliers for Black Friday.... right out of the package you just knew they were cheap and not made well.... I never got that impression on the Fat Max... they may not be as polished as the Knipex, but they still feel good enough.
If you can truly buy Knipex though for near the same price, get the Knipex... from what I can see though online, the fat max look to be more in the $8-$15 a plier range where knipex is more like $30-$55
In that same range though is the Tekton, made in USA, and those are really nice. However they have the dipped handles, as opposed to the dual material molded ones like the fat max or NWS have....