1.) Lag. The usual culprit.
2.) The Riot Shield isn't perfectly in front, but actually a bit to the side, so stray bullets from spraying will most likely hit you and enough of them can kill.
3.) It can depend on the game mode. If it's Free For All, Team Deathmatch, Kill Confirmed, or something like that, it's not really recommended except for fun. But on things like Drop Zone, Team Defender, Domination, and such, it can actually come in handy at times.
4.) If you're just running with it, then your feet will be exposed. When you come across enemies, you need to crouch down and it will block your feet off. Otherwise, you need to look down a bit, but that will end up possibly exposing your head too much to fire.
5.) If you're moving forward when it does, then the animation has you moving "into" the explosion when it explodes (as the second it explodes, it's no longer stuck to the shield and is simply in the air), thus the explosion is technically happening BEHIND your Riot Shield and on your body. So if you get stuck with a Semtex, then you'd best quickly move backwards instead of forward or try just stopping and see how that does.
Otherwise, all I can suggest is using Blast Shield with it.