I dont think its a question of strength,more that the egine to gearbox alignment was not correct so the drive plate was being flexed too much.One customer of mine was replacing them every six months,even with two plates in place - till he sorted the alignment issue.
I have to say that although te engine pulled well and was not sluggish,it was far from refined.Sitting at the lights in drive is like sitting on a jack hammer ! They really needed more work on the engine mounts and a shallower profile sump.They fitted blocks on the front axle to stop it going too low - meaning the car could never fully get down to access level,and yet the track rod still wore holes in the sump...
The Mazda conversion was a much better job,smoother etc and the electrics had been more tidily done.Had that been done to an auto it really could have been a worthwhile conversion.