Nextcloud is a steaming pile of barely maintained donkey dung one should not approach with anything other than a 10' pole while wearing a full Hazmat suit.
I've run Nextcloud for years primarily for webdav file sync and calendar+contacts. It pretty much just works. That's not to say there isn't any jank, but it's mostly if you try the more obscure apps e.g. I've set up the phone track feature and it worked for a while, but I've had trouble with it recently. Probably fixable, but I just haven't spent the time.
I would say it is the core component of my self-(vps)-hosted setup and easily the most valuable.
What are its cons? I used it years back and worked fine; did it transition to the dark side or it's a matter of bugs? I'd use it only self hosted of course. Alternatives?
I use it at home just fine. Granted, I only use it for files and I’m not using the office apps from it.
Can you go into what you issues you saw?
Last time I checked it (a few years ago), it was quite buggy and crashed often for me (self-hosted).
For example, what I remember, there was a module to move photos from Google Photos, but it always stopped at 10% photos without any errors. Then there was also some document suite module and it would hang my entire machine regularly, for some reason (had to restart it).