Oh, I definitely don't think Revenants are unbeatable. I just think that games involving a Revenant would not really resemble what I like about 40k.
Like, what if they came out with a new model, "Cosmic Coinflip Machine". When it activates, you flip a coin. Heads, you win the game. Tails, you lose the game.
There is a very effective strategy that you could use to beat the Cosmic Coinflip army half the time - just let them activate it! It's definitely not an unbeatable army. But... yet...something is missing... oh yeah, it's "playing 40k".