I was moping the floor up today with Red Death, I really think it's the best weapon in PVP. It's a 2 shot kill when landing all head shots, and a 3 shot most of the other time, plus it heals you upon kill, so good.
I've went up against the Mythoclast twice, and both times my team has won, and the player wielding it has had a K/D of like 1.5. To be honest it doesn't seem to kill much faster than any other AR in the game, but I fully expect a hard mode end game gun to be slightly above the rest, it is a loot game after all. Quit taking it head on and you'll find it is possible to beat it. Red Death is similar, I can't just rush into a fight with an AR guy, I have to distance myself, and use cover to get an advantage.
Maybe my opinion would change against someone who was really good, I don't know, but I feel like the vast majority of people who have it, really aren't going to be able to punish you with it, if you know what you're doing.
I gave up on IB though, so disappointing. People who have put the time in to level up guns should immediately feel that difference against low level players, not be killed just as fast as normal crucible with a Shingen, lol. They either have to go all in with balancing the game to make it as competitive as possible, or embrace what makes it different from every other FPS MP out there. Half assing both just leaves you feeling frustrated with the game at times.