As someone who's had the pleasure to own and wrench on several Citroens (A SM, DS, and several 2CVs) the subtle details of the SM are incredible.
It is hands down, to this day, the best highway car I've ever owned. Rides nicer than a modern S-Class. Great seats. Fantastic visibility. Nimble. So long legged on the highway they just pull and pull.... And that Italian V-6. Sounded incredible winding out.
Regret selling that car more than anything else I've ever parted with. I will snag another; just finding one in the US that hasn't fallen to the tin-worm is hard. I may have to fly across the pond, and send one home.
I've owned both the SM and an Eldorado of the same vintage. The Eldo is by far the fastest front wheel drive car of 1970.
That said, the SM was quite a car. I bought one wit a blown engine, and swapped in a 2.8 liter Ford V6. It was a lot of work, including making the Ford engine run in reverse rotation. Not as fast, but it made the car quieter and a lot more tractable than the Maserati V6. Mine had a 5 speed transmission.
My understanding at the time, 1973-4, was that Citroen's could not be certified here because they didn't use DOT brake fluid. They suspension and the brakes used the same hydraulic system, and it used mineral fluid similar to AF. The fact that they had an adjustable height suspension had nothing to do with it. Later Lincolns had adjustable height air suspensions.
Rolls-Royce used Citroen suspension and brake technology, but they used DOT brake fluid just to get around this problem. The rule was relaxed in later years, and R-R went to the mineral based fluid.
Cars that are failing in the marketplace often blame government rules when they leave the US.