The rails only failed on engines that were overheated or the oil was never changed in. Aside from that, the rails would hold up past 120k miles with the new tensioner.
The difference was in the way the tensioner behaved on initial start up. The old tensioner relied on a spring to hold tension on the bow rail while the engine was trying to produce oil pressure during cold starts. The spring wasn't strong enough and many of them failed and the engine jumped time.
The new tensioners have a ratchet device that will not allow the piston to go back, therefore the likelihood of chain failure is greatly reduced.
I installed one in my 16v at 118,000 with no ill effects.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS