I actually went through the Allegheny county “newcomer tax” just some months ago.
It was a bit of a strange process to appeal (I lost; my house is very weird for the area).
While I do see the benefit for not raising taxes so consistently for long-term owners (and could definitely see gentrification-esque effects) it does seem like a pretty obvious - if bitter - pill to swallow if the area is going to have any chance of continued growth.