Well, first of all, I can run the sorts of apps directly on Sandstorm that I might otherwise have to order AWS or GCE style hosting for. I can publish a website out of this as Drive is actually retiring that functionality.
Also, unlike many Drive apps, where I have to share my information with a third party provider, or "trust" the app, Sandstorm doesn't let my apps betray me. Privacy is inherent in the design.
But the key thing, is if I don't like my host, I CAN LEAVE. I am at the point that I loathe storing my data with Google. I love Gmail, and I wish I could take it with me and put it on a server run by a company I trust. But I can't do that. With Sandstorm, I can. I can run the app I want on the host I want. I have control of my app, my experience, and my data.
Google ruins my experience on a weekly basis. They change things without my permission that breaks my workflow and makes it hard for me to go about my day. I KNOW my Sandstorm hosting will do what I want it to do.
I use the manual filters. The supposed intelligent crud is a hindrance and I've largely shut it off. Gmail is a great web mail client, nothing more.
The big issue, is your much talked about data and heuristics are unreliable. Since Google changes their software or algorithms without my permission, I can't rely on any of them.