The way everyone seems to use it, in my experience, is giving the client credentials to manipulate the full database which is absolutely insane. Obviously this is not good practice but every single parse project I've gotten thrown into already did this and it was a significant amount of work to move it to be more secure and NOT do that. I think the way it's created makes it very, very easy to make bad security choices.
It's also basically a RESTful CRUD. That's mostly it minus some, mostly minor, bells and whistles. You can already do this with about 20 different open source stacks very easily and you're not stuck using this parse technology.
I can't articulate just how much trouble parse has given me. I even took an app and re-wrote its entire backend away from parse significantly faster than doing a handful of updates.