After rebuilding over 30 in my short career, I would say that they are strong as hell. The only time I see them fail is if they are neglected or abused. I have seen it all...I do have some die-hard service customers with well over 300k miles that have the original tranny.
The one thing that is a fact is that if something goes wrong, in many cases, it is just plain less liability to replace than to try to repair because it has been not serivced properly or abused.
I hope this helps.
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM