The pricing seems reasonable, free and then goes up to $200/month once you've gained decent traction. Though it's more than double the cost of a VPS that could support 1 million requests and a 1GB database, and that could make all the difference for an ad-supported app with razor thin margins. https://www.parse.com/plans
1) security. what is preventing an end user from opening a js console and going crazy on the db? How can you ensure consistency when the user has access to the code?
2) Growth. if you choose parse as a platform, the lock in seems like it could cause significant pain down the road.
seems great for toys and demos. I would not run a functioning business on it.