I'd try avoid it too for the same reasons Steve mentions, but saying that, I have landed, not taken off from the roof of mine a few times to avoid blowing dust up around the motors, and have had full normal control each time, maybe though it's not something to do continually as it could throw the calibration out one day.
Taking off - if you are constantly accelerating seems to avoid getting dust into the motors.
I've launched and landed on the tonneau cover on the back of my pick up truck. No issues. A little prop-wash from the air coming back from the back of the cab.... I've done this without issue with all my different "drones"...
I don't USUALLY do the car roof top thing, BUT... one morning I was getting ready to fly and a lady was out there taking her Great Dane for a walk and he was off the leash. I decided that was a GREAT day for the car roof top take off so he wouldn't get to it...she then hung around for 20 mins...so I had to land on the car again to keep it out of his reach. Everything went perfectly smoothly