Our best engineer spent 2 weeks full time updating our codebase from 7 to 8 using rector (the project was already running PHP 7 but still had a lot of deprecated code from the PHP 5 era which didn't work on PHP 8). That would be eternity without Rector.

I've previously used that to do something. Fell flat on its face. But the fact they literally had to create a tool to solve this problem confirms my point.

I use it all the time and it works great. You need to know what you're doing.

So again, you seem to be confirming my point that it’s not fool proof.