I have never seen any clipping issues, even when they could still be placed on walls.
They go through anything that doesn't have collision, so namely "undergrowth" which includes a fair amount of the rubble and rocks on the ground. It's not a problem with the American AP mines because they're quite tall, but the S-mines can be almost completely obscured if you put them inside a rock.
Regarding mines on stairs: utilize engineer kits. Isn't that one of the things you people are complaining about? Engineer kits, with their wrench and mine markers, are actually useful. You could also just crawl over mines without triggering them.
This is impossible with a lot of mine placements. People know of the issue where you can't get near mines on stairs and exploit it heavily. It's a big problem on certain flags like the central flag on Bastogne or 16p Mt. Olympus since they have an ammo box in the flag zone and its possible to completely (or almost completely in Bastogne's case) wall off the flag zone with s-mines by placing them at stairs and ladders.
And it's not unrealistic to make disarming s-mines easier. While yeah, you can't realistically disarm them by shooting them, they also shouldn't be a huge threat if you see them. Even if it is in a place where you can craw under it, realistically you'd just step over it or something. The cost of having to stop and go prone under it is pretty huge if you're trying to assault a flag.