The 600 operates its windows, seats, trunk lid and doors on a high-pressure hydraulic system that commenters agree is the single biggest hurdle in ownership. The most cited failure points are the pneumatic air suspension bellows/bags and the master control switch block in the driver's door, both notoriously expensive; by contrast the drivetrain, suspension, leather and wood are described as comparatively straightforward.
The hydraulic 'comfort' system as the defining complexity
Mercedes-Benz 600 Grand Mercedes (W100) · mechanical