100% agree with Soulgazer in regards to the fact that Combusken has been good the whole XY NU generation. As most people know I like to use fairly obscure stuff, I saw the fact from the very beginning that combusken with baton pass was going to be extremely good. I think it was either my 1st or 2nd team that I was able to see the power of swords dance baton pass combusken. Its ability to so easily get up attack and speed boost for fairly bulky and powerful yet slightly slower than the typical sweeper stuff like kangaskan, feraligatr and klinklang was really appealing and was so easy to get high on the ladder (which doesnt mean much but eh). The only "real" problem for combusken back in the day was fletchinder, which had priority acrobatics to handle combusken, and even if you ran bulk up > swords dance, fletchinder could swords dance to more quickly boost its power. I am guessing that was why combusken did not receive as much love. Now, as seen by the recent replays of the top players in the tier, it is just so easy to have one poke that cannot 2hko or even ohko combusken before it sets up at least 1-2 bulk up and at least 2-3 speed boosts. However, at the very worst, combusken can set up at least 1 speed boost, which is really all some pokes need.
Combusken really has everything going for it. It has very acceptable bulk for its job, as it can easily take neutral or even super effective hits from the best pokes in the tier like shiftry, seismitoad, spiritomb, granbull, gurdurr, and easily get boosts no problem. This can be somewhat said for gorebyss too, however, the bulk is only there from a white herb, and cannot take special hits as well. Combusken is also neutral to stealth rocks, which is a big big reason why combusken is a threat and ninjask is almost not an option. Sure, ninjask can perform a similar role with swords dance, especially with the defog buff. However, ninjask requires a fair amount of team support for it to be effective (as well as gorebyss), while combusken really needs no team support at all to do its job and can really fit onto any teams. The benefit of just getting one speed boost after protect and baton passing to even a wall can make or break a match. And the fact that combusken can do this multiple times without really any resistance (remember, if a poke switches out on combusken, protect will not work, so you will essentially get two free speed boost) makes this really the ultimate support poke. However, probably the best benefit of combusken, and something I really should of thought of at the very beginning of the meta, is getting those defense boost from bulk up.
Bulk up on combusken imo is really the breaking point for this poke in the tier. Its ability to get even one defense boost makes it difficult for pokes that you would think would be able to handle it 1 v 1 cannot. A couple examples (coming from 248 hp/ 152 def neutral defense nature, not sure what standard is but that is what I use, but there is leeway nevertheless) are that Max attack rhydon/golem cannot ohko combusken, after a bulk up, with eq (even with rocks). Feraligatr without life orb cannot kill combusken after a bulk up boost with waterfall + aqua jet after rocks (feraligatr has less than a 2/3 chance to kill with life orb, but life orb feraligatr is extremely powerful). Adamant banded sawk cannot ohko with any move after a combusken bulk up. These are some of the most intimidating and wall breaking pokes in the tier that even cannot really handle combusken. Of course, at the same time getting these defense boosts you are also getting attack and speed with is absolutely unfair. I guess in comparison, gorebyss can also live all these hits after a shell smash and white herb activation, but you almost need max hp and max defense to do so. And, of course, you can only only get +2 speed (combusken can easily get +3), and the poke that you baton pass into is going to be taking a massive hit from these powerful pokes because you do not have the defense boosts (and even moreso without white herb). The fact that combusken can pass defense boost into pokes that are already bulky, like the ones I mentioned before but also stuff like slurpuff, xatu, samurott etc make this an even bigger threat. Of course the added attack and speed really let the poke be scary and destroy teams. But another added benefit is that these defensive boosts will immensely help you take all of the physical priority in the tier, like sucker punches, fake outs and mach punches without the fear of dying to these hits, which cannot be said for any other baton passer.
Yeah sure, there are pokes that can stop combusken from passing big boost, mainly, special attacking pokes that can outspeed combusken after 2 speed boost and hope to ko. But there aren't many pokes that can do this. And the fact combusken can set up on all your other pokes really isnt that helpful anyways, unless you run clear smog magmortar...
if that was the problem, then Ninjask would not be shit. It's more than just BP + Speed Boost. It's complimented by Combusken's typing and decent defences with eviolite. If Combusken is banned, and it is still broken on Torchic / Ninjask, we can relook at it. But right now, BP + Speed Boost is not on the table.
This in a nutshell. Now before I say I want this thing banned (oops), maybe it is fair to allow whatever dropdowns occur in the next couple days determine whether this thing needs to be gone, however, I don't think much will be able to stop the terror of combusken.