Composters cannot be pushed via pistons

If they were pushable then we could replace cauldrons with composters
Cauldrons only output signals 0-3, while composters can do 0-8 and can use hoppers to change the signal

Absolutely. We could potentially even port over the cauldron functioonality from bedrock without needing to break complex redstone functionalities. All they have to do is just replace cauldrons with composters.