When I was there at aws (left about a decade ago), us-east-1 was considered least stable, because it was the biggest.

I.e. some bottle-necks in new code appearing only _after_ you've deployed there, which is of course too late.

It didn't help that some services had their deploy trains (pipelines in amazon lingo) of ~3 weeks, with us-east-1 being the last one.

I bet the situation hasn't changed much since.