- Rock solid/reliable/"good"
- Not changing
And they are completely independent!
- Windows 95: It's definitively not rock solid. But is it "stable"? Sure! You will have a hard time finding something more "stable". When was the last time it received an update/changed? It's "stable", but not "stable".
- RHEL: Rock solid? I don't really have a lot of experience with it, but let's say yes. And its whole business model is about changing as little as it can. It's "stable" and "stable".
- Fedora: Rock solid? Let's say yes. Does it change? All the time. It's not "stable", but it's "stable".
- Linux in 1991: No idea. But I'm guessing it was crashing all the time. And it surely was changing fast. It was not "stable" and not "stable".
"Stable" software can be not "stable" on purpose. You may want to avoid changing so much that you may want to keep the bugs, people rely on that buggy behaviour!
Not "stable" software may have shitty QA and not be "stable"; or it may be so well tested that even changing all the time, it never fails.
If I have a contract with the government for a software that needs to provide a service for the next 5 years, I surely will target RHEL X. Not because I think it's specially good, but because I don't want to find myself in court about whether the contract said I need to keep supporting them every time they update the OS. I will deliver something working better or worse today and once accepted... it will keep working the same, bug by bug, relying on the same CentOS X bugs, in 5 years because the underlying system has not changed a bit.