Here are the only two issues I've found with lag screws, and neither is the fault of the screw itself:
1. I came upon one of my virgin campmates HAMMERING the lag screw into the ground. That was the one time during the burn that I kinda yelled at someone (more from shock than anything) -- he was just uneducated. He'd bent the screw somewhat from this technique. We will have to train our virgins better next year
2. Without a guy-line attached, the screws can be practically invisible. A few of our campmates had to leave early, so they took down their structures while I was away. We'd guyed down some of their shade with lag screws, but they didnt know how to remove the screws (or where the impact driver was stored). Therefore, they just removed the guy-lines and packed up, leaving the screws about 1/4" below the playa surface. Luckily we could see the outline of where the walls had been, and the depression in the playa where the screws were, but you couldn't really see them on their own. We checked very thoroughly, so I'm hopeful that we didn't leave any behind, but it would have been nice to have a little flag/string/something on them. Ideally they'd be taken out as you take down the structures, but with a big camp, this can be hard to guarantee. A metal detector would have been nice... though probably overkill