I have only ever once had my army called OP, and it was during my first 3 months playing the game against this guy playing Imperial Fists, while I was playing Chaos Space Marines.
He had set up his list so that he could have a ton of marines storm up behind a line of three Centurions, and had no transports to speak of. As he was playing Fists, his ultimate goal seemed to be to find some nice spots to camp out and start gunning down threats. He also had a squad of Sternguard camping a building, and his HQ, possibly a Chapter Master, running up with the marines.
My list, being limited to what I owned, consisted of two squads of Chaos Marines, one with a plasma gun and heavy bolter, the other with two plasma guns. They were meant to be backed up by two squads of autocannon Havocs, and a Tzeench Daemon Prince with the Burning Brand would lead them into battle. And by Warp if that little guy didn't do just that. Then there was a squad of Warp Talons deep striking in to help the Daemon Prince land a nice charge. I think I had something else in there, Chosen perhaps, but it really doesn't matter.
Oh, and this guy also was cheating! Since I didn't know the rules all that well, and this guy, having fully painted his models to be Fists, talking about how much he loved their lore and their additions in the codex, and being a veteran player who had been in the game for over 10 years, claimed their Chapter Tactic was that every single bolt based weapon was twin-linked, all the time, no matter what range.
Well it sure ended up not mattering at all anyway, because Prince Killgore (the name I gave to my Daemon Prince) nearly tabled him in a few turns. My marines hid behind a wall until his army had been reduced to a single 10 man squad of marines, with Prince Killgore still slamming away at the Sternguard, which now had his HQ in it. Since the mission was The Relic, my guys just kinda walked up to it, picked it up, and began walking back to their wall. The Havocs were able to kill a few marines here and there before Prince Killgore got to them, and for the most part they just watched the carnage unfold.
And the best part of it all; at the very end, he starting shaking with rage as Killgore finished off his HQ with ease, and screamed "I ****ING CAN'T BELIEVE HOW OVERPOWERED CHAOS IS! DID G.W. EVEN TEST THIS?" It was then that my friends came over, one of which began laughing as he saw a lone Daemon Prince on the opponent's side of the table, and piles of dead marines on the other being packed up by an enraged Fists player. The other just asked how it went. I told them that at first I was scared of their Chapter Tactic, but then the Daemon Prince was so powerful on his own that he was able to wipe out whole squads in a phase. Then, the guy who was laughing stopped, and his face turned to the most wide-eyed expression the man could make, as he turned slowly to face the raging loyalist.
"You're cheating with Space Marines, and calling a brand new player with a single Daemon Prince overpowered?"
The rager immediately stopped what he was doing and turned pale, and quickly tried to cover up his lies. My friend pulled his copy of the codex, and slammed it down next to the now-cheater, open on the Chapter Tactics page. "Really? I sure don't see a line that says you have twin-linked all the time! Do you? Please, enlighten me!"
The player cried "But the Daemon Prince is still overpowered! He killed almost all of my army like it was nothing!"
"That's because you were using a stupid strategy that you bolstered by trying to cheat. Next time, actually play the game by the rules, and build an army that follows them."
The cheater then stormed out of the store, and we didn't see him again until recently, trying to sell off some old Imperial Fist models.