I think Yhwachs power is like Nicolas Cage in Next (crappy movie, interesting premise), where he can see into the future two minutes, but can see every possibility around every possible decision he can make.
And it branches, so for example if he goes two ways, he can see the possibilities for each of those two ways. Within his two minute limit of course.
Yhwach probably has this same ability but with a much longer time span, and with Mimihagi being immune to precognition.
Haschwalt probably had a short time limit or a limit to the possibilities he can see when using the Almighty.
So the terrifying thing is that Yhwach saw every possibility up to his goal, after Mimihagi was absorbed, and picked the best possibility. So everything is set and can't be stopped because he saw and picked what to do and it's happening as he saw it. That is the epitome of being fucked and the heroes have no idea.
I can see this being stopped if Aizen makes some kind of illusion where he tricked Yhwach into seeing his goal is accomplished, because he is probably so arrogant that he wouldn't think to use the the Almighty beyond that. This would open up a future that he didn't see.
Or maybe Mimihagi or some other being immune to precognition will appear. Or maybe Urahara will have some device. Or maybe Haschwalt will pull a Gin like betrayal. Or maybe something totally unexpected, but I doubt it.