Salvatore, for what's worth, every time someone complains about how one shouldn't use project X because it has too many open issues, I just point them to Redis as a counter example: Works wonders; majority of issues peripheral ;)
Open issues aren't a problem. Anything with a sizable user base will have a continuous stream of issues. That's the mark of being popular! People only complain and open issues about things they actually use.
A "bad sign" would be increasing or stagnant issues with no resolution. Say if a project has a single maintainer and said maintainer decides to stop all work on it. There's plenty of examples of projects like that and if you're choosing a library or backend, you need to know what you're getting into.
(For the record, Redis doesn't fall into that category at all!)
If issues are being opened faster than they are closed, the number of open issues is going to grow without bound. That's just basic queuing theory.
It's also the case that the average age of open issues will grow without bound. (As the number of open issues grows, eventually you get to a point where more issues are untouched in any given period than are either opened or closed.)
So, it's very easy to get into the state where there are lots of stagnant issues. It's going to happen automatically unless you do something to fix it. Something needs to happen so that either fewer issues are opened or more issues are closed.
