As others have complained, canonical and a lot of extra overhead (like mount an entire FS for every snap running). I tried it and got rid of it quickly. I do use flatpacks but really only on my gnome desktop (arch) due to the ease of the gui software center being right there. (EDIT: This is how containers work, I get that, it's just a lot for system utilities or small apps)
Are you using nextcloud locally or facing public? I run nextcloud on a homeserver but for anything public facing I run in a docker container on it's own bridged network served up by Caddy proxy container. It's no where near "hardened" or full proof but it's something better than just facing a snap at the world tbh.