"The pto shaft is a very weak item to rely on as a safety device..."
It sure is and I don't read anywhere where someone was suggesting it as such.
The mechanics of getting a mounted shredder deck vertical to ground level while cutting is the odd thing here. I've cut a lot of ground, but, I've never come anywhere close to having a mower deck rear-up as described in the posts above. The PTO shaft would seem to give a good deal of warning of this "problem" pretty quick as it buckles and binds and joints fail as the deck comes up.
One net positive with having a direct, non-live/independent PTO is that when things get hairy, applying the clutch stops everything going on.