In simple terms before the new law you had to go to court to get a patent reviewed. Now you can file a request with the patent office and skip court. Which makes it cheaper and easier to fight bad patents.
The counter-argument that might lead one to disagree with this decision is that the Patent Office reports to the President, so it is subject to the political motives of that branch.
For example, let's say, a major democratic donor receives a software patent on a novel ML algorithm useful for predicting voting behavior. Under inter partes review, the President (or his subordinates) could invalidate that person's patent if challenged regardless of the evidence. If inter partes review were found unconstitutional, they would need to convince a jury that the algorithm was, in fact, inappropriately granted.
All inter partes review decisions are subject to judicial review by the Federal Circuit. The standard applied by the Courts is called "substantial evidence." To dumb down the standard (it is only slightly higher than this): The board cannot simply make shit up on the way to its decision.
Your hypothetical would probably fail under even more deferential standards of review. God fucking knows it would not survive the Federal Circuit--I'm happy to link plenty of cases where they told the board that its invalidity decision was not supported by substantial evidence.
You're right that IPR is judicially reviewable and I'd agree that my example would probably not hold up to review if clearly politically motivated and contrary to evidence.
Someone who's worried about this might argue with you that to have a patent invalidated, then the invalidation subsequently overturned is not the same thing as not having a patent invalidated in the first place, if only in the eyes of the market. I don't think it's a meritless argument, though it's not one that sways me personally.
No, because this decision wasn't about whether the process was good or bad, but whether patents are private property, which was the point made by Oil States International.