The Benz's of the late seventies through the eighties seemed to have the worst a/c systems. That said you have to divide the "a/c systems" into two categories, the refrigeration system itself and the Automatic Climate Control portion.
The refrigeration systems themselves use standard components much the same as most all cars and as a result their reliability is the same as most cars. The downfall of the refrigeration portion is their lack of cooling capacity. Remember Germany does not have the same climate as Florida or Texas.
The Automatic Climate Control is the source of most of the MB a/c problems. The cars that have manual a/c control in lieu of the ACC are pretty darn reliable albeit still plagued with marginal capacity systems.