If they have steel inserts, well that's certainly a lot more re-assuring than if they were straight alloy, but I'd want to cut a few apart and find out just how much steel is there and if it is just like a helicoil/thread, or if it has a flat section as well so that the 'clamping' surface is steel too. If it had that all in place i'd at least say 'well it could be a lot worse' :) sort of thing, but to be perfectly honest - given the fact that _if_ they let go or failed, they likely wouldn't do it whilst sitting in your car in the driveway, but rather they'd let go under the most stress - braking or cornering most likely (cornering probably more so, due to direction of the stresses). And the harder the cornering forces, the more likely the failure.

And I reckon it'd be fair to say that if you lose a wheel mid corner, esp an outside one, that rolling the car would be practically guaranteed. You might not be killed, but it's possible, and the car would be wrecked beyond repair as well.