The DMR is the easiest gun to use and it's a mistake that 343i made it a personal weapon. If you really want to play competitive you should use the Battle Rifle and the Magnum/Plasma Pistol otherwise a good DMR user doesn't = a "competitive pro" player.
Every game is overrated because the Internet hates everything and wants anything that's going up to go down. Like a true villain.
I'll put it this way: It is absolutely more difficult to 4shot someone with a BR in Halo 2 (even offline) than it is to 5shot someone with the DMR in Halo 4.
You are objectively wrong. The Halo 2 BR has more aim assist and magnetism than the Halo 4 DMR. The BR from 2 has no spread, so landing all hits is easier than with the BR from 3 or 4, even without factoring in the extra help from the game.