Yes, I have faith in PF certificates. Can there be conflicts? Yes
You are focused on particular issues that are shade dependent for value and those issues are a tiny subset of what the PF certifies. One of the more common conflicts you will find though are stamps with old certs (think 30 years ago roundabout) that are subsequently found to be reperforated.
Everything and everyone is fallible. Overall the PF does a fine job and people respect and value their opinions the World over.
Confucius say "do not burn house down because one wall has crack".
So, I have faith in PF certificates, too, but not because they are infallible but because they drive the market. Said another way, I might think an unused stamp I have in hand is a 70b, 70c, 70d, or 78c —but I would not be willing to spend 70b70c, 70d, or 78c level money without a PF (or in some cases PSE) cert accompanying at the time of purchase.