Unfortunately Dropbox cannot be used on the iOS mobile app, this a limitation of iOS. Nothing Obsidian can do to fix that. I use Dropbox for all my cloud stuff and I have still decided to go ahead and purchase sync for the following reasons:
- In app version history and ability to rollback version history for 1 year! (much longer than dropbox)
- End to end encryption, I trust Obsidian and their service for keeping my most personal of notes safe and private
- The future roadmap includes other stuff like sharing and collaborative features I believe
As far as the service goes itself I have found the sync works seamlessly without any issues between all my devices (I use win 10 PC, Macbook Pro and an iPhone).