I'm using a gravity feed ATO. Works great so far. I think the issues that can go wrong are the same with any ATO. Float valve gets stuck instead of switch getting stuck... overflow in sump (I have more than enough room to accomodate the 5 gallon reservoir I have)
I can imagine if you had a 30-40 gallon ATO or direct connection to your RO/DI than it might be a bigger deal...
I just didn't see the need for an extra outlet, an extra pump, wiring, etc when gravity and a float can do the same thing...