You're more likely to catch the chainring before the roller but that never happens anyway . Doesn't get covered in mud either as it's out the way of the tyre . Even if it did get hit and the roller manages to push against the side of the chain it has already been over the chainring and the chain isn't under tension as it travels to the rear so the chain would stay on , any deflection as the chain comes off the chainring is lost along the chainstay length . Even if it's a bit rough it would still pedal . But bend the rear mounted tensioner and the chain would lift off the sprocket at the rear .
The rear wheel just slots in as there is nothing in the way like with a hanger mounted tensioner . If you wanted to get really technical having the tensioner in the center of the bike low down is a better place to add the extra weight than hanging it off the rear of the bike .