It is probably a better battery than the Pegatron or Foxconn POS that was originally installed so you should probably count yourself lucky. I have had some devices with Li-Ion batteries that still work fine after years of service and others that crap out after a short time. Sometimes it is the type of service that is partially to blame, but often it is the quality of the battery.
Cell phones place a wide variety of demands on batteries depending on the user and the way that the phone is used. That is one of the reasons why users have such varying experiences. However when a manufacturer does what Apple got caught doing... it is a good indicator that there was a problem with a whole generation of batteries.
I am not a electrical, chemical, or quality control engineer so I would not be qualified to say why the failures have been so widespread that Apple was forced to take secret measures in an attempt to hide the problem. But I think that it is safe to say that replacing one of their crappy batteries with one from a reputable supplier has probably saved you a lot of trouble.
The batteries in those older iPhones were made by Amperex Technology Limited, the same battery maker who supplies Samsung phones. As of the iPhone 8 and X, LG supplies the batteries.