In my sample size of a dozen or so, I think Amazon/ebay batteries for tools and cameras are 100% as good any OEM battery I’ve ever had. I’ve never had a failure. Failure means that I need to go back to my charger and grab a different battery a few minutes earlier. It’s low consequence.
Years ago I worked construction with a rope access group. The dudes at one of the training facilities mentioned that they ran across some look-alike rope that had a core made of paper. It was likely some other wadded up fibrous material but had fractional integrity compared to the stuff we were using. That was fairly high consequence.
Bikes are somewhat in between. That’s the point I am trying to make. There is a place for knockoffs. I guess you have to figure out what that place is for you.
I’ll add a thought experiment. Let’s say you have a $4 carbon handlebar that fails on a group ride or even a race. You walk away with a couple scrapes. People in the wreck have thousands of dollars of damage. Some are in the hospital. Maybe someone has injuries that will mean they’ll never ride well again.
Now, how do you feel about the few dollars you saved on a handlebar made of cheese? How do the injured (and their lawyers) feel about you riding with them on such equipment?