YMMV obviously but having used CyanogenMod for the past few years on various devices I've found it to generally exceed the stability of vendor-provided Android. Not to mention the better user experience and more rapid security patching.

"Custom roms never run stable from my experience and that is why I have stuck with Google Nexus devices in the past."

Coincidentally enough, the custom ROMs for the N4 and N5 are ubiquitous & surprisingly stable. My N4 running CM 10.1.3 has yet to crash or freeze w/out my fiddling with Privacy Guard(been fiddling with it for 2 years, became daily phone only recently). The Sailfish OS ROM has come a long way and is still actively updated. Sure they're dated & SFOS is somewhat limited(and trust isn't quite on par w/ Maemo) but what else is there? Yeah, Neo900 was an admirable reboot attempt, but roadblocks have put them even further behind the curve.

Funny that, my experience is quite the opposite.

Nexus 6P (Marshmallow); any time I lost phone signal the messaging app would get itself stuck in a tight loop until it had to be force stopped. You'd think they would have tested that on a brand new device..

Cyanogen Mod has been great in the past, as you say, to extend the life of old phones. Quite stable too.

I have a custom ROM on my oneplus 2, it's smooth like butter.