I agree in that, YOU "feel like ORD is always the easiest hub to clear Polaris upgrades w/PPs"
But I Absolutely Always Disagree that ORD is always easiest. It is Never Always ORD.

First, what do you mean by clear? At Gate? At Booking? In between?

I almost exclusively only book if PZ>0. For Europe, I'd say EWR, IAD, ORD are about equal. Don't even consider DEN or SFO since I live on the east coast. In terms of "impossible", I've considered IAH but have never found anything.
Going to Asia, you're mostly going via SFO or LAX so you'll naturally clear there more than other hubs with alot less flights.
Going to South America, its mostly through IAH so I guess IAH would clear more.

If you're talking at the gate, That depends more on your status and booking class and luck.

The route matters too, some routes have more flights or more people buying J.
I find for europe, I've mainly been going via FRA, LHR, BRU. The nice thing about europe is that it is easy to connect and not have to spend too much time in Y.
Asia much harder going out more than a few weeks.
Pre-Covid I would see South America and India frequently but not lately. I've never seen anything for south africa.

For day of week, I have found most of my upgrades has been Tues-Thurs
In terms of planes, there's the ratio of J/O/Y.

If you read through the PP Q&A thread and/or the PP Success thread, it should give you a better idea.