Blaziken can just toss out High Jump Kicks and Flare Blitzes and do substantial damage to even some resists such as Salamence and Latias, and it'll often outspeed the opponent after the first Speed Boost so that it can land the second hit before they have a chance to react.
But regular Blaziken -won't- often outspeed an opposing scarfer after the first speed boost because it's only 80 base. Mega Blaziken has a much better shot at that, but if the argument is that -mega- blaziken is unstoppable then banning the mega stone would be the issue. Also, as mentioned in an earlier calc, intimidate salamence still exists and takes unboosted hits like a champ. And the only Gyarados that Blaziken even -can- 2hKO without boosts (but with stealth rock) is full offensive.
When a faster Scarf user comes in on Cloyster (and there are plenty of those), it either switches out or dies.
But that's usually the same situation for blaziken.
Really a lot of the arguments for a broken blaziken seem to run into some significant 4SS. If Blaziken is running Dual Stabs, it only has two more slots to choose from between coverage, protect, swords dance, and Baton Pass.
If a Blaziken has Swords Dance and Baton Pass, it -can't- protect to outspeed the scarf switch-in, and it is forced to leave. It also doesn't have any coverage, so it can be forced to pass or switch and give an opponent a free turn (a free sub for Flash Fire Chandelure, et cetera). And the things that are going to be switching into it that are outsped after one boost are things that generally don't rely on outspeeding their targets to begin with, so they don't really care if it passes speed. If you're not running swords dance, you're only passing speed. And the meta is shifting more toward bulk in general than in Gen V already, so fewer things care about being outsped. Without Protect, you don't have an answer to a faster scarfer switching in, you switch or die. Without Baton Pass, you can't pass your boosts if something pops in that doesn't care about your stabs. Without coverage, you open up more pokes who can come in and force you to make a sub-optimal decision.
Talonflame is everywhere, not just because of Blaziken but because it is an incredibly good check against Hyper Offense in general. It's not like Talonflame was #3 in usage in October just to stop Blaziken, it was there because Priority Brave Bird just rips holes in the 'speed + attack' metagame of Gen V. People aren't running Talonflame and Azumarill because of Blaziken since they're both used more; Talonflame and Azumarill have nothing to do with any overcentralization on Blaziken's part. They are just a part of the metagame that needs to be considered when determining what something can accomplish, just like how being killed by Bullet punch was such a big deal in Gen V when Scizor was around every corner.
Also, forcing out Blaziken also does help rack up damage, given that it damages itself with any HJK miss or with Flare Blitz Recoil attacking switches, while also damaging itself with recoil, while also resisting no entry hazards.
You could argue that Blaziken is broken -because- it can do many different things, and if you don't figure out what it is going to do before it starts it can ruin your day. But that was the same argument used for banning Garchomp back in the day, which clearly didn't stick. Blaziken might be worse than Gen V Garchomp, but Garchomp had the cute touch of not -needing- speed boost since it had Dragon Dance and a trolly base speed score to beat out scarfers, while also being immune to thunder wave and resistant to rocks, and existing in a meta where sticky web would never be a concern. Different stabs, different pokes, and Baton Pass mean that obviously it's not the same situation, but given the entire Garchomp saga I would think that actual discussion and testing would happen before just jumping on 'it should need to be banned because it's difficult to counter.'
I also find it odd that everyone keeps saying 'as if a blaziken user would fire off HJKs at switch-ins with ghosts around,' when ghosts are basically everywhere; especially with ghost's newfound amazing neutral coverage with steel's resistance going away. Flare Blitz has lower power and gets extra recoil (on top of hazards and life orb), but messing up HJK once can just kill you, since it can easily cause enough 'prior damage' to put you in revenge range of a lot more priority than just Talonflame and Azumarill.