Well , they're not (quite) as bad as Fiats , if that's any consolation .
Seriously , the 1950's models ( Pontons , Adenauers ) weren't so bad , most surviving unrestored examples tend to be substantially sound . However , by the time the next generations came along ( W110,111,112,113,108,etc) the rot had literally set in and all of these models were known to rot away underneath , even though they might look respectable on the outside . This persisted with the 'New Generation' models (W114,W115) and also with the original S Class (W116) - all of these cars rot in the same places : wings (inner and outer) , sills , floorpans , boot floors but can appear outwardly not too bad !
It was only with the introduction of the W123 series that Mercedes began to get their rustproofing act together ( these cars don't tend to rot just so badly as earlier models , but are not immune ) and further improved it with the W201 and W126 models which tend not to rot terminally , likewise the W124 are pretty good .
Sadly , after this , things seemed to downhill again with the W210 and some others , before being addressed in recent years .