When (not if) it breaks, you're screwed: no way to get access to the data, the client is completely opaque and there's no logging or a way to reset it. When it stops syncing, or they “handle” a network or server error by silently logging you out, you won't know until you miss a meeting or use stale data.

The main reason I pay for Dropbox is that simple model. I have a full copy on every machine, and if my internet connection fails, they go down, or something breaks I can just copy files like normal. If I see something odd, there's a full log & the ability to revert any changes.

There are times where boring is good and your core personal data is one of them.