Most who seemingly gripe here do so because they own a name brand car that ONCE had a better overall reputation for being very durable and long lasting. I won't say "totally troublefree" but the older RWD Volvos had relatively minor and less costly problems compared to todays Volvos. Not only have I repaired them for over 20 yrs but my old RWD, high mileage Volvo is STILL a very decent and troublefree car that I drive everywhere w/o problems.
The true test is the test of time and mileage and being that you just bought your S80, you don't yet know what'll happen. At the dealer that I work for, most of our S80 customers are not exactly "happy campers" when they need to keep bringing the car back for neverending problems, new problems (not repeats) so even if it's covered by warranty, the need to keep bringing these cars back gets very old for these people and they don't seem happy, most say it'll be their last Volvo. When the warranty has expired, the repair costs get very high as nothing is exactly inexpensive for these newer cars.
As for safety, many other brands of cars today are equally safe so Volvos claims to that are now also exagerated.
As for comparing an S80 to a Mustang, not really an apples to apples comparison because Mustangs were never made to be a luxury car nor known for comfort. I'm sorry to hear about your back problems but I'm also not surprized that you didn't find a Mustang to be confortable either.
When everything's working as designed, yes the S80s are very confortable and nice cars to drive, no doubt about that.