Despite a few other names in DW that strike fear into the heart of an opponent upon Team Preview, Blaziken has to be the most frightening. Forget about actually sending it out, the mind games it creates are enough to cause an opponent to play erroneously, enough to the point whatever strategy they had planned crumbles beneath their feet. I personally haven't had much of a problem with The Flash's chicken offspring, but that's only due to running a team overspecialized to killing it. Even then, I've been swept quite easily by it after Exca dies or it gets to the point of outspeeding Scarfchomp. I rarely get lucky and have out Techniloom already at +2 when it comes in. Also, Ken with Air Ballon basically fucks any attempt Exca had at killing it.
Take this for example: Opponent sends out Blaziken on something it can force out, in my case lets use Deo-S since that's happened before. My Exca is still alive and Sand is up. In the turn I switch to Exca it SD's (or protects, but why a Protect against Deo-S idk, most deo I see run superpower), that's +1 speed. I need to break the Ballon to EQ it, but since it used SD it now protects this turn... that's +2 speed. Finally, an opening! But wait, Rock Slide is 37.42% - 44.37% max, 49.67% - 56.62% with SR up. At that point of the second turn Exca being in, it either fails to ko or is destroyed by one of Ken's STABs. If it gets to the third turn and Exca is still alive, it no longer matters. Ken can now outspeed it and most everything else in existence, and is +2 in Atk.
I mean, sure you can get ballsy and SD on the turn it SD's. But you're fucked if 1. SR isn't up 2. Flinchhax doesn't kick in. And because Ken has, uh, teammates, and because Rapid Spinners exist, betting on getting a perfectly timed SD and praying Rocks are up is a hell of a lot to bet on. To make things worse, nearly everything I've said is extremely situational. Not everyone and their mother carries Exca, or him along with his partner in crime, Garchomp. I have taken down Blaziken with Keldeo... but that's probably the most unreliable way to kill it, as an unboosted HJK is 96.6% - 114.2%.... which is an 81.25% chance Keldeo doesn't live to fire off a Pump. Forget if you're not running Pump, or it misses. And forget Keldy if it kills Ken, it's basically dead weight after that hit. ...Annd chances are if you're sending out Keldeo against it, it's already either raped half your team or you had nothing better to send out in the first place. Again, all of this is situational. In general, Blaziken is going to come out and do whatever the hell it wants, when it wants, how it wants. SD & Protect as needed, and then either sweep with one of the best STAB coverages in the game, or Pass it's ungodly boosts turning just about any poke that gets them into a nearly unstoppable monster.
As Jimbon mentioned the only pokes that resist its STABs don't like taking them boosted; and... I've actually seen Ken get +4 Atk fairly easily, a number of times, so forget Type Resists once that point has been reached. It's killing whatever it wants.
And Baton Pass... forget it. Between BP & Air Ballon, formerly any reliable way to kill it is flushed down the drain. You got it nearly crippled? It just passes +2Atk/+2Spe, or some other inane boosted level of those two stats (+4atk/+3spe ;_;) to a teammate waiting in the wings (garchomp anyone?) , gg.
I could go on for days with this, but while Blaziken is nearly unstoppably broken in its own right... Baton Pass is what takes "nearly" completely out of the equation. You could all but mangle Blaziken... doesn't matter, it passes its boosts to a new, healthy Sweeper and all your work is for nothing as this new threat rips apart your Blaziken-battered team like a hot knife on butter.
If there's a #1 threat in DW, it's Blaziken.
If there's one poke who's banishment would improve the overall health & balance of the meta, it's Blaziken.
There are a few things against Ken too, Nachos has listed them quite nicely, better than I ever could of. But, considering everything Blaziken can do offensively combined with BP, those Cons very well may just not exist in the first place.
If Ken lacked Baton Pass, its shortcomings would be suffice to leave it where it is.
But that isn't the case.
All of this is just personal experience from facing it... but in this lowly user's opinion, Blaziken needs to go.