I have two major problems with it, what it's written in and how it manages files.

It's written in PHP. I'm not complaining about the language... I started as a PHP dev and fully recognize why an establishment might use it. The issue I have is PHP is always a pain in the ass to get configured properly. Of course there's always docker implementations, but even those haven't been constructed well imo.

Secondly, the way it manages files is absolutely terrible. Now I haven't really used it in about a year so I don't remember the specifics, but I now have a folder on my NAS that is pretty much not removable due to what I believe is an abuse of ACLs. What I really need is just something with a web interface that can sync with my other devices without imposing its own file/folder structure. At least with my experience in the past, this is a pain with Nextcloud.

Can't speak to your first point, as my only interaction with Nextcloud php configuration has been to comment in/out various php modules. I don't have too many modules though, so maybe that's why I haven't had any configuration issues.

Regarding file management, the working solution for me has been to mount any external storage with a fuse mount ( NAS, dropbox, etc. ), and then just use the Nextcloud "external storage" option. Other than Nextcloud's general slowness ( I'm running on an underpowered server ) I haven't had any issues.