Full disclosure: They reimbursed me for one bad card I bought when I proved that it was trimmed, but only after months of arguing and levels upon levels of proof. I spent way more in time getting my money back than it was worth, but that wasn't the point. All of this has colored my opinion for sure.
So here's the thing. PSA is a complete and utter scam because they're not effective at detecting alterations even if you eliminated the favoritism. But it doesn't matter because there are too many idiots with money to burn. I've divulged myself of every card I have that derives value from the grade. My T206 Eddie Plank PSA 4 isn't going down no matter what happens to grading because I have provenance from a well-known collector who had it since the 60s and there are less than 100 surviving. That's not manufactured rarity. The random PSA 10 Rookies I had of players I liked for various reasons are all gone.
I don't know how you can have a brain in your head and look at the threads on Net54 or Blowout and think it isn't all a house of cards. If it isn't irrefutable proof of trimming, it's encapsulating crap like this:
10 seconds of Googling would tell you that's a fake. Or having eyes.