After developing my own roguelike, I actually started to admire Nethack for being so stable despite having so many special behaviors. You could play version 3.4.3 for years and not find a single bug. I think that it tells a lot about the code.
If you don't find any single bug while playing 3.4.3 for years, it might tell more about your style of playing than about the code
But seriously, to this day the admin of NAO has to regularly recover games because of crashes and you have to remember that the version on NAO has fixed all known crash bugs that are in 3.4.3. Granted, most of those crashes happen if you take the game to the limit, for example doing excessive pudding farming. I myself found several crash bugs by ramping up the monster generation and looking how stable is the game when creating a monster might fail (not very, but it's easily fixed).
There are lots of bugs in 3.4.3 (no wonder, it's a big game) and you can easily make the game crash. For example when saving a game while riding and wielding Sting, the game crashes while loading the game or - one of my favorites - just throw a potion at iron bars.
The official bug list on nethack.org lists about 500 bugs, on nethackwiki there are some more. Is that much for a program that was released 10 years ago?
You could say no because most of those bugs you can workaround or avoid by not doing some stupid ascension tricks (although NetHack offers itself for doing such thing and it's part of its appeal) and there are very few bugs that can hit you without you doing anything (for example a monster with a potion of acid on the drawbridge falling into the water because another monster zapped a wand of striking that collapsed the drawbridge might lead to a crash).
OTOH you can say that as the last real changes to the core code was in 3.3.0 which means 1999 that this is quite a lot. The release notes for 3.3.1, 3.4.0, and 3.4.1, talked about hundreds of bug fixes (3.4.2 and 3.4.3 "only" about dozens). But of course the real problem here is not that there are bugs but that there are no bugfix releases anymore.
You shouldn't look at your roguelike and feel bad that it has bugs. If you had hundreds or thousands of volunteers that tried to find bugs and because of the seclusive nature of the DevTeam we can't really say how many of those volunteers also wrote patches for fixing bugs. If you had that, your roguelike would also be much more stable.
We often overlook the fact that NetHack didn't get big and popular just because of the DevTeam but also because there was a great community that helped the DevTeam tremendously.