The things you'd normally use Nextcloud for is not the sort of thing that I would ever trust to be hosted for free, there's bound to be a catch (whether that is the service just disappearing over night, your data being collected and used for some nefarious purpose, etc).

It's definitely worth either self hosting, or if you're not able to, paying for some VPS to host it on.

That all being said, if you really want to go the route of free hosting, Nextcloud apparently do list some providers on their website.