There are “known good” parts and there are “new” parts....... and they aren’t always the same thing.

I never just assume new will be good.

With something like an assembled head, there are a lot of different pieces that all have to fit together in a certain way to not have problems with it.
It’s no different than buying a computer or a washing machine.
Just go on line and read the reviews for just about any item you can think of and you’ll find a pretty large number of defective units.

Auto parts are no different.

One of the differences between a head and a microwave is ...... you can bring the head to a shop and have it checked out before spending a bunch of time and $$$ on other items that are needed to complete the job.......so that you don’t end up with a defective unit leaving you on the side of the road........100 miles from nowhere .......at 3am.
When the microwave dies, you just bring it back to Walmart....... no biggie.

When you have a new head gone through by a qualified shop, and get it back with a clean bill of health..... then you have “known good”, and “new”.