Anyone who has played Search & Destroy, or Search & Rescue knows that the Riot Shield + C4 is the most overpowered thing in this game (as far as search goes at least). I don't play many other game modes, but I do play a lot of Search, and I run into this nearly every game. It completely ruins the experience, and I'm honestly surprised it hasn't been fixed yet. Keep in mind that these game modes limit you to only one life per round (unless you're rescued, of course).
1) the riot shield can have a portable radar, which gives away your position so fast if you don't spend 3 points to counter it with Off The Grid.
2) C4 + Danger close leaves you without any chance in hell. You can unload on riot shielder, and unless you get extremely lucky, you're going to take a C4 to the face before you even get a hitmarker. Blast Shield is nearly useless against them as well due to the power of a C4, I am constantly being killed with only one C4 even when I use Danger Close. it gets even worse with danger close.
3) The riot shield's animation is also severely overpowered. A riot shielder can throw a C4 AROUND his shield, only taking his hand out from behind shield. You no longer put the riot shield on your back to throw a C4. This means that you can toss a C4 out at someone without even putting yourself in harms way.
4) Strong Arm + C4 lets you throw C4s around corners, over walls, and across a pretty large area with ease. One of the best counters to a riot shield is distance, Strong Arm almost completely eliminates this advantage. If you're close enough to aim at the riot shielder, you're close enough to die from his C4.
5) Reflex. Putting reflex and a pistol (usually akimbo) on your riot shield class allows you to keep the advantage even when you run out of C4s. I understand that Reflex was made to switch weapons quickly, but it's just insane with a riot shield on. There are many cases where I've been shooting a riot shield, and they were able to switch weapons and kill me faster than I could get a couple bullets on them. It's an insanely fast animation, and you have to be insanely lucky to survive it.
These are the major reasons why riot shielding is overpowered, but there are other instances that make them unfair. Please let me know what you guys think, and spread the word if you agree with me. If you don't, feel free and comment and let me know why you don't agree. I'm curious to see what everyone thinks.