I am sorry, but what are you talking about?
Manchu was my first country i played in the last patch, and I easily defeated ming on Ironman with no problem.
Started as the northernmost tribe, became a tributary, DoWed and annexed two other tribes (while u are a tributary ming won't defend the other tribes against you). Became Manchu, Then annexed Buryatia, and upgraded it's goldmine. While still a tributary annexed half of Korea (which was probably useless anyway so you can just skip them), and the steppe lands north of China proper belonging to Mongolia and the other pink-colored country (Korchin, if i recall the name correctly).
Then waited untill Ming pursued the first reform, their authority dropped to around 35. Started refusing all of Ming's tribute requests, which lead them to break my tributary status (i didnt want to lose 1 stability). After they did, they were hit by the event "unprotected northern border" which hit their stability and dropped their mandate to around 10.
I declared war and just waited with my banner-armies in the steppelands where i easily defeated Mings armies thanks to the severe debuffs China was suffering due to the lack of mandate, but also because the horde government gives neat bonuses to fighting in the steppes, and because banners are OP. Since your armies are generally smaller, Ming would attack them, instead of avoiding them, but because of the 3 aforementioned facts Ming would lose with insane casualties (he'd usually be losing 10 times more troops than you). And then banners don*t even need manower to reinforce....
So yeah, Manchu is actually OP when played right. Dont buff or nerf them, please. They are perfect!