Behr cooling system wasn't up to par with Texas summers. Germany has yet to produce an air conditioner to match the American Southwest's heat although it doesn't help that the Mercedes wagon has so much glass, either.
Maintenance by a decent mechanic is a must for these vehicles, particularly replacing the timing belt at regular intervals.
Small electrical gremlins, particularly with the accessories seemed to be a problem, i.e., rear windows, rear wiper, dash lighting, bulbs, and the occasional alternator.
Despite the high cost to maintain this vehicle properly, it was a beautiful highway cruiser, with much to recommend it. HUGE hauling capability, but quite slow with maximum passenger load (7). These cars do not like to sit, and seem to run better when driven hard and often. The W124 is made with high quality materials (rubber gaskets, plastics, wood, carpet), much better even than later models, and I am confident they would outlast any mechanical aspect of the car.
The W124 wagon was fast, smooth, stylish, completely useful and reliable. What more can you ask for?
The inline six is a strong engine, let down only by the cooling system.