Also adding a +1 for this - the auto-tracking support on Heap is critical for early-stage products - often times we need to retro-actively ask questions we won't even know we'll have and heap lets you track events without instrumenting them

I'm going to add a +1 for this -- the major reason to use Heap is the auto-tracking support. Not having to know beforehand what events to track/instrument is a major win. See https://heap.io/product/autocapture. Note that Heap has a react-native-sdk but this is not supported under managaed Expo due to missing native modules.

What benefits does it have over amplitude? Why heap specifically over something like Firebase?

Good suggestions for features such as this (i.e. not core hardware or device experience) give use cases on why it is better than other options.