I'm not expert in parse or stackmob, so I can't compare it feature-by-feature. But there are two general advantages of LeanEngine:
1. It's open-source so you have all the freedoms that come with OSS. You can tweak it, add features, fork it, etc..
2. It runs on your AppEngine instance. So you have full ownership/access to your data. Also AppEngine is proven to be scalable, has tons of features, has SLA and is (still) cheaper than comparable cloud solutions.
Second,
