I try to answer:
NextCloud is a generic host to store files. Joplin as many other applications can use NextCloud as target, but speed will not be optimal, especially for large repositories (i.e. the user has added many files, images, etc to the notes).
Joplin cloud is a Joplin optimized target solution that one may rent to get a supported target for storing the notes. Obviously this solution will propose the best performance for Joplin.
