It's sitting in Simperium's database and you're locked into their platform once you decide to use it. Maybe there's an export, but that's irrelevant because all your apps in the wild and all your users are relying on this service to move data around.
If Simperium gets hacked, you get hacked. If Simperium goes down, you go down. If AWS goes down, Simperium goes down, and then you go down.
Say your app gets huge, and Simperium can't scale (and really, "half a million users" is nothing for a free service; that's not any meaningful scale and not a number that can really be cited as a signifier of ability to scale) -- what happens then?
Say you outgrow Simperium, and it's now costing you too much. What happens then?
The above isn't really about Simperium, but really about all backend-as-a-service services. They're great for starting out, but if you actually have a successful product you're going to need to be running your own backend at some point.
If you get too big/successful/etc and things start dying and you end up like Friendster before Facebook decides to buy your company, you're going to look back and wish you built your own backend and knew what was going on behind the scenes.