Two words: Speed Boost. Let me dumb that down for you... one of the best/ most overpowered abilities in the game. In case you didn't know, it boosts your Speed by 1.5x each turn, and Blaziken is fortunate enough to have access to this ability. 1.5x Speed basically makes you faster than all of OU, and faster than an amount of common Scarf Pokemon as well.
Speed Boost combined with Swords Dance can transform Blaziken into a deadly sweeper in just one turn, and it's especially problematic if you give it that turn to set up. With access to great STAB attacks like High Jump Kick and Flare Blitz, is is capable of demolishing teams. Speed Boost Blakizen was banned from OU and banished to Ubers as a result. OU simply can't handle a Pokemon with so hard to revenge kill and so hard to switch into. There are definitely Pokemon that can stop it, like Azumarill, but if Blaziken is forcing you to use certain Pokemon, that's proof that it limits team building options and makes the game unbalanced.
Blakizen also has other tricks, like Baton Pass to give boosts to something else if it is threatened and Protect which essentially gives it a free Speed boost.
