Mostly engine problems which are easily excused because it is a French built car, no offense to the French. Not to mention its not easy task finding someone to work on these cars, I can only relate this car to a Rubik's cube, it looks simple, but turns out to be a total conundrum.
This car is rather bizarre, the gages are nearly impossible to read, other than that the car is an excellent ride. Very innovative and sleek styling. So many features as well, the headlights turn with the steering wheel, ingenious. If you have the means of picking one up, I recommend you do so, they are also rather rare.
The engine is an Italian Maserati engine, not a French one... no offence meant.
Maserati and Citroen engineered the V6 together to go in the SM and the maserati merak. the merak's engine is tuned for more power, but a less supple ride. if you needed you could tune your SM engine the same way as the Merak's and get similar performance figures, but at the sacrifice of that silky smooth French/Italian feel.