You could not install Nextcloud on Sandstorm. Sandstorm has an app that used to work with Nextcloud's mobile app, but Nextcloud makes a very poor Sandstorm app because it is a monolith, instead of being granular.
(Personally I also find Nextcloud incredibly sluggish, but it is quite popular so it must work better for others.)
Was a pretty good idea but there wasn’t any sustainable business model/funding and it was also pre docker era which would have made things more secure/containerised and easier to maintain.