The attraction of dropbox and similar was having an off-site backup. No security concerns for me as all the Gigs of material is family history related gleaned from public domain sources - so happy to store on a server that I have no control over. The value to me is in the many hours it took searching and collecting...

Correct me if I have this wrong - but I believe NextCloud uses your own private server for storage - so not quite as good a backup against data loss in the event of a recovery required after a disaster...