The Riot Shield itself is perfectly fine, working pretty much exactly like its predecessors, yet complainers of the C4 combo try to constantly place the blame of the combo being "OP" entirely on the Riot Shield itself, yet not on the C4 (which is doing the actual killing) or the Perks (which they themselves, like the C4, probably use on their own classes).
Even Danger Close, by itself, isn't that big a deal when you only have a very limited supply of explosives like you do in Ghosts. That's what made it OP in MW2; the fact that you had One Man Army to refill launcher ammo (to spam under-barrel grenade launchers and such) or Scavenger to refill explosive Lethals (Frag, Semtex, C4), essentially giving the user "infinite" explosives in a single life.