After going back through this thread I think there's three main things in the favor of demons that have been either overlooked or missed entirely.
First there's the issue of magic. While it's not the ultimate weapon of doom in the demons' arsenal it is something that can throw any match in their favor when used properly. The Impervious to Energy spell alone would rend most (if not all) mechanoids weapon meaningless for the spell's duration. Furthermore, the evil mechanoids are forbidden from not only learning magic, but using it to open a rift as well (SB2, page 52). This brings me to my next point...
Dimensional travel. Every demon returns to Hades when it dies to be reborn, and can only be killed permanently in that dimension. That's going to help the demons tremendously. The demons will always be able to retreat to another dimension if whatever fight they're in goes badly for them even if they die. The only thing that would help the mechanoids in this case would be getting an "inhuman ally" to create rifts for them to go through. Even then, such a being would have to be non-humanoid/biped for them to be allied with the mechanoids (SB2, page 87), and would be considered an enemy if ever found to be helping humanoids/bipeds. That description includes deevils as well. The deevils might be able to get by convincing the mechanoids that they're not humanoid/biped for awhile, but it just takes one deevil (or a captured demon for that matter) to make the mistake of letting the mechanoids in on their true form before they all know about it. The mechanoids are a species that communicates psychically. One deevil makes that mistake and it gets passed around to the other mechanoids locally, eventually to a runner, brain, brute, overlord, or oracle and from there it ends up going to a brain pool. Once that happens evey mechanoid knows the truth and they reenact the
Order 66 scene with the deevils. This kind of dove tails into my last point.
Everyone else in the Megaverse. Demons aren't stupid. Demons likely know which humanoid species the mechanoids ticked off the most, and are going to be able to either recruit from those species or trick into fighting the mechanoids on their own (Kittani are the first to come to mind). Other species might be able to be guilt-tripped into fighting them ("Well Mr. Cosmos-knight. You could stay and fight me, but there's a Spider Fortress about to crush that colony of settlers over there.") Either way, mechanoid threats are likely one of the few things that get people to team up with the demons to fight, kind of like what the US and Britain did with Stalin against Hitler in WW2.
you some might think you're a 
but you're cool in book 
Phase in 
