[pikelet]

I think you're overstating it a bit. There were definitely features that couldn't be implemented due to MV3 limitations rather than because the developer chose to leave them out.

https://github.com/uBlockOrigin/uBOL-home/wiki/Frequently-as...

[frollogaston]

Hm, I can't tell how serious these limitations are, but there are a lot, and this one stands out:

  replace=, can't modify the response body (full support is only possible with Firefox MV2)