Speaking from the perspective of warfare, unit efficiency and the likes:
There was never a point after 1.20 where banners/manchu would be legitimately useful.
Prior to 1.29 you had banners that were strong, but insanely difficult to micro properly and just extremely tedious to deal with.
Then in 1.29 Paradox looked at that, and decided that since banners were hard to micro and not really worth their price too often, that they should nerf them to the ground, leading to the current bonuses that banners have.
Then 1.30 came along, bringing in a much needed improvement to the UI, allowing you to at last reasonably recruit banners, but... it's still simply not worth the effort
-50% reinforcement is NOT pretty easy to deal with. In simple terms it means that whenever you use your banners for anything, they are effectively useless for a solid few months before they can be used again. While this is somewhat manageable if you are defending your own territory, when it comes to doing anything on hostile land, where further penalties to reinforcement rate and attrition apply, it just becomes a nightmare.
And obviously you have other downsides of using banners; you can't really merge them, they are useless for assaults...
And most importantly: while +5% disci, lower maintenance and less manpower cost looks cool on paper, and very much is cool for the first few decades... it's simply not anything amazing going into mid/late game, and that isn't even including the high cost of actually getting a decent number of banners
In general Paradox has systematically struggled to make special units that are actually worth anything.
Marines have always been almost useless. Janissaries are so bad that recruiting them is borderline nation ruining
The only usable special units in the game right now are raiputs and streltsy; but it's not like they are amazing either, they are just.. nice