FWIW, the game demo showed that you can mega evolve and attack on the same turn.
Sure, but that still means a slow Pokémon takes a hit before its stats get boosted.
I never liked using legendary pokemon because it felt cheap, but there are legendaries who get placed in UU tier too. Saying "legendary" doesn't actually say much.
That's an artifact of the bizarre way that Smogonites play Pokémon.
Maybe bizarre is too extreme, but it's certainly not how GameFreak thinks the game is meant to be played. It seems natural, though, right? Six Pokémon fit on an in-game team and Single Battles are the de facto standard format, so 6v6 Single Battles should be the normal way to play, right? And yet, you'll notice that every Battle Tower-esque format in the game uses 3v3 for Single Battles. All the console battling games do too. Huh.
When you have six Pokémon on your team, you have a very good chance of having a Pokémon to "counter" whatever your opponent brings out. Teams are built around this very idea of covering all your defensive bases. As a result, 6v6 Single Battles are crazy switch-fests. You and your opponent dance around each other, trying to whittle down the Pokémon or two that stand in the way of your sweeper, which you then bring in and clean house. In a 3v3 battle, you can't afford to do that. But in 6v6 battles, this tends to skew in favor of Pokémon that fulfill one specific role very well, leaving more well-rounded Pokémon in the dust. That's one of the two reasons that certain legendaries don't see play in OU.
Enter Stealth Rock. Because switching is so incredibly prevalent in 6v6 Singles, entry hazards are very powerful. Stealth Rock is the quickest to set up and also the strongest, and as a result almost every team has a Pokémon to set up Stealth Rock. Stealth Rock just destroys Pokémon that are weak to Rock-type attacks. This means that several entire types of Pokémon are rare or non-existent in OU. (It also reinforces the whole switch-to-a-counter game, since there are fewer bases to cover.) That's the other reason that certain legendaries aren't used in OU.
Entei is a perfect example. In a 3v3 environment, Entei is a beast. It hits like a truck, can take a beating, and is a good bit faster than the average Pokémon. In Smogon and Serebii's 6v6 game, it can't even cut it in
UU. It's a sick joke.