You mean, you “own” copies of your data, but unless the person who controls the interface with that popular happy workflow that Everyone Already Knows How To Use lets you store it on their server, a lot of people who might otherwise be willing to collaborate with you will go “eh” and give up, and therefore, since so many marginal things require that to get anywhere, you can still just lose completely if you don't agree to the GitHub Terms of Service?
I imagine you didn't mean that part primarily, of course—but it's another thing to be aware of. There's a lot of potential for SaaSS interface lock-in—I think this is much worse for users with less technical inclination or less mental slack, but even in the realm of “people who are already developers who use Git”, this becomes a large friction source. And with more-social smaller-world flows, the coordination-control aspect has a stronger grip.
That said, it's also worth looking at how the “cloud” model is in a way reifying the folk model many users were already using. People thinking of their documents as being “in” (for instance) Word, and having no idea where they are in a filesystem or what that is in the first place, is already seared into the cultural memory of people who were family tech support in the 1990s. The default consumer cloud model is an easy extension and solidification of this: now it totally is “in” Google Docs rather than being a leakier folk-model weakening of accessing your your hard drive, and now Google Docs is accessible from anywhere. Everything is like you expect! No need to panic when your machine breaks, just get a new one and it's all still there!
And those upsides are real in their own way. But a number of downsides which are also real are less obvious and immediate.