It could be apples and oranges here of course. If you have a decent aerial on a radio that does not move, especially if it is in some form of HiFi, FM quality is predictable and generally quite good. But even in this case, the stereo decoding introduces hiss (try hitting the mono button on your tuner when listening in a quiet environment, and seeing how the hiss disappears). And you must remember that many of the commercial FM stations use dynamic range compression and dead space elimination techniques to boost the quiet parts of the music and make hiss less noticeable. Try listening to Radio 3 or Classic FM if you want to make comparisons.
In a car, there is all sorts of interference, especially when the car is moving. There are drop outs as you move, especially in built-up or hilly areas, and cars are not good environments for electrically sensitive equipment (yours may be well shielded and suppressed, but you cannot control the rusty 20 year old Fiesta that pulls up next to you at the lights!)
In some cases, DAB can eliminate this interference. If you get a good enough data stream, other interference can becomes irrelevant (it's digital!), and transmission and decoding hiss disappears. But more often than not, the same interference that degrades the FM signal will also damage the digital signal, and when the DAB receiver does not get a enough of the digital stream, it either burbles or just drops out for a couple of seconds, whereas FM may still be listenable.
My guess is that a lot of the people who say DAB is good and FM is bad listen to FM in the car, and DAB in the home, whereas a lot of the people who say that DAB coverage is bad are probably trying to use it on the move, or just in areas of crap reception, where FM degrades more gracefully.