I never had any (major) problems with Nextcloud yet.

I just have following "conflicts" with it:

• It doesn't follow the "Do one thing, and do it right"-philosophy. It tries to do everything at once. File upload/ sharing, media management (NC Photos), RSS, mail, calendar, contacts, and much, much more. I mean, it's damn convenient and works pretty fine, but nothing is great. For example, Immich/ Photoprism is way better than NC for photo management.
• There's a lot of abandonware, or buggy/ unmaintained apps. For example, my "News"-feed looks completely broken for months now.
• The performance isn't good. I mean, the "server" (an old thin client) isn't fast at all, but the loading times and responsiveness is just awful. The file upload also takes ages, even from the same network.
• It feels bloated. I think, if I would be more into selfhosting and had more time, I would search for alternatives and split all the NC features I use into their own services, e.g. one for file upload, one for document management, one for managing my photos, an own RSS client, and more.

But, as I said, the ease of use and amount of features is still great. I don't want to spend three weekends just troubleshooting my server and searching for/ installing dozens of individial services. And for that, it's good enough.