If your pto driveline has not changed sound, it is probably ok. In most cases, if everything survives the hit, it's life has probably not been shortened. There is'nt a predictable cumulative effect of high momentary loads unless they are high enough to cause a failure then and there. Shear pins and other "soft" external drive parts can fail progressively, but internal drive parts are usually hardenend and don't deform plastically under load. If the load is too high, they fracture and the game is over.
Over the years there have been several times when my mower has run into something that slows everything down - if it is a pile of dirt or something dense, but not hard, and if the deceleration takes place over 2 seconds or more, there has never been internal damage. Maybe a pto shear bolt would break, maybe the pto shaft would tie in a knot, but the tractor was always ok. The loads it sees are not that much higher than the max rated horsepower puts on it. Everything gets stressed, but nothing inside gets very close to breaking.
The risk of breakage goes up fast as the load increase becomes more sudden. If your engine goes from 2500 rpm down to stopped in less than a second, that's getting near breaking something. Also, if the shredder gets into a big rock and starts delivering heavy hammer blows to everything, the instantaneous forces are huge and gears can crack and break.
On some shredders, the blades are heavy a make a huge racket when they tie into something, but they can pivot so the actual impulse transmitted back through the driveline may not be as bad as it sounds.
Take care, Dick B