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From all the tokens the tokenomics are by far the best in stem. The weakness of stem is that general people dont care that much about science especially not if its not a fun video :)

Is every publisher's author reward is burned?

No, but if you do not post on https://stemgeeks.net 75% is burned.

What is the APY of STEMM?

I am not sure, this changes based on how many miners are staked and price. I think right now it is around 10%.