If culverts and concrete box culverts will wash out, it’s hard to picture a concrete slab not being undermined.
I think the biggest issue with the culverts and box culverts is the amount of debris (trees) that get caught on the upstream side. The weight of the water pushing against them just blows them out. My thinking on an at grade crossing is by the time the water gets deep enough to wash whole trees down stream it will be several feet above the crossing and they'll just slide on over. Sure, you'll have to wait until the water level recedes to get across, but at least there will be something left when it does go back to a normalish level.
