NH TN 55, Kubota B2320 & RTV 900, Bad Boy Outlaw ZTR
I don't know anything about the engineering aspects, but there may be safety issues. I don't think the PTO guard on my tractor (NH TN55) could accommodate a slip clutch.
I think space is at a premium on many tractors and therefore the slip clutch at the implement end of business. BUT.....I gotta wonder if you were to disintegrate a slip-clutch or even lose a bolt or part from the clutch....it could hit the operator if mounted on the tractor side. (?)
Much less likely to stick the wrong piece of flesh around the limited slip PTO clutch as it is unshielded and usually partially obstructed by the three point hitch mount on the implement.
Reminds me of the very young son of a former co-worker that caught his coat on a shaft that was not shielded on a sliage wagon and he was killed almost instantly.