Outside of it's document editing, I find Drive's iOS app to be pretty half-assed. Dropbox can handle anything you throw at it much more reliably, and typically has a far better built-in viewer/player. You can't play .m4a files in Drive, for example, or search within a .pdf. Sure, these things can be offloaded to other apps, but I find this makes managing my phone's storage a lot more cumbersome. I also like how photos/videos get their own tab in Dropbox's iOS app.

I wish the iOS Drive team spent half as much effort on these details as they did shoehorning their own UI conventions into iOS.