[noduerme]

Have to chime in with others... FPM has definitely been the most reliable and easily tweakable system I've had in almost 20 years of PHP work, particularly since I run a lot of legacy code off the same server in different versions of PHP.

I feel like problems with monitoring may be down to lack of proper logging or experience. It's not like FPM is going to leak more memory than your code would in other running contexts. It doesn't "go down" since it spawns a new process for every request. My cold read is that there's something wrong with the code you're running, not with FPM.