The SDK's were slow to update, the Java App Engine doesn't support Java 8 yet the Python and/or PHP version are behind a bit I think. There's a bunch of bugs which haven't been resolved after a long time. The instances were underpowered for the price, Java ones took a long time to start up which compounded issues with the auto-scaler.

I used it a little bit a few years ago and despite the issues with the original platform I'm quite interested in the new Docker based managed-vm app engine platform, where you can build your own images and have much better control, and compute engine pricing.

When they went out of beta/preview, the prices skyrocketed to absurdness and many people had to shutdown their app because of everything being coupled to their specific API and the notice being to short to adapt.

So yeah, don't invest into something when you don't know what the final pricing will be.