I've never heard a negative complaint from an Uber driver either (sample size ~10 drivers). I have never used Instacart because I'm not in the Bay area.

I would also add that it is much easier for Uber to roll up their charges and markups into one fee (only one service provided and consumer generally has a wider range of acceptable cost) than it is for Instacart to do so (multiple products, easy to estimate cost of a loaf of bread).