The problem at Dropbox seems to have been that there was no cohesiveness to all the products. Paper, Passwords, Sign, all seem to have never been truly integrated into a single experience. Each one felt like it was trying to have its own identity.
Ironically, just within the last year Paper has gotten much more integrated into Dropbox as a single UX. And… it's significantly worse: slower, clumsier, harder to navigate. (I don't think there's any inherent reason those had to be correlated; it's just that Paper has clearly been destaffed a lot in recent years, so naturally any new changes will tend to be less polished.)