I've got Halfords roof bars and used them for bikes, boards etc... Never had any concerns with them, so imagine genuine ones would be even better. Actually, one concern about all clamp on bars is that no matter what locks they have on them, you always seem to be able to pull the clamps away from the roof.
I've fitted a Westfalia tow bar now, so that I can use a bike rack clamped to the ball.