No one is going to say that Speed Boost + Baton Pass is broken on Ninjask on the same level as Combusken. Ninjask Pass is for the most part incredibly easy to stop, since it sets up on almost jack shit, is vulnerable to priority, has poor offensive potential, has bad typing, and is hopelessly frail. Combusken is....pretty much none of those things; literally the only thing it doesn't have over Ninjask is Speed, but Ninjask's speed is overkill anyway and there's this little thing called Speed Boost, don't know if you've heard of it, that helps to remedy Speed in a pretty significant way.
However, that alone doesn't make Combusken broken as hell. We'll need to see the many ways Combusken can do its job. Bulk Up is already receiving all the attention, since it makes use of Combusken's typing and Eviolited bulk to easily snag boosts. Personally, I welcome any Pokemon that can help put Shiftry in its place (as well as Spiritomb), but when said Pokemon can start setting up on Pokes that can hit it SE (i.e Seismitoad, Sandslash, Pelipper, Uxie), then I can't help but feel that the Pokemon might be a bit too good at its job. Suddenly it becomes a matter of what Combusken can't set up on, rather than what it can. Offensive teams usually don't give Combusken much leeway in terms of setting up, but even then they may have a Pokemon or two (Shiftry, Spiritomb, Pawniard, Cryogonal, Accelgor) that gives Combusken the opportunity it needs, and a good majority of the time the setup bait's teammates cannot stop Combusken at that point. This does not even account for defensive teams, who may find their whole damn team unable to stop Combusken and co. in time, and more often than not enable Combusken to pass more than once.
So as a result, the meta is encouraged to go even more offensive. Even if you don't consider this an unhealthy metagame change, do consider the fact that Combusken can just as easily go the offensive route with Swords Dance. Speed Boost can feasibly be an offensive tool shown by users of higher tiers, and Combusken is no different. Between its strong STABs, Eviolited bulk, and constant increase in Speed, offensive teams can very well find themselves overwhelmed by the Combusken wrecking train, especially when you consider that most offensive teams pack the aforementioned 'Combusken setup-bait'. Combusken can use Protect to block Fake Out and resists Sucker Punch, making it truly a challenge to bring it down with priority as well. Oh yeah, and if Combusken does find itself backed into a corner, it can simply pass its Attack and Speed boosts to a teammate and let them take over. Swords Dance is not even the only way Combusken can go offensive; dual special STABs are a perfectly viable option, since both have well over 100 BP to leave a mark on offensive teams off the bat, and Special Combusken is simply content to pass just Speed. Let's see your Rhydon stop Combusken after it has eaten a Focus Blast, eh? If you guess Combusken's set incorrectly, it is likely to get away with passing again, or simply wreck your team itself.
Combusken has sheer versatility between all of its sets, yet they can consistently accomplish their main role(s): pass Speed, along with other such boosts. Even Gorebyss must be wary about Smashing: the threat of fast Scarfers or Liepard's Encore can make it waste its White Herb, meaning subsequent SmashPasses will have their recipients enter battle with lowered defenses; if Gorebyss does not pack White Herb to begin with, it must worry about damage both for itself and its recipient, which limits its range of setup targets. All Combusken has to do to pass Speed is Protect, which does not pose any risk to Combusken at all.
That said, is one Pokemon being grossly consistent at its job too much for the tier to handle? Possibly, considering the amount of Pokemon that benefit from Combusken's support, and Combusken's potential to threaten defensive and offensive teams alike. Even relatively obscure Pokemon such as Gogoat and Octillery will improve from Combusken's support, much less already great powerhouses such as the usual Kangaskhan, Feraligatr, Xatu, etc...