From balance perspective janissaries right now are essentially trash and using them is borderline nation ruining.
- They require you to spend mana, which prevents you from getting ideas/slackens/technology
- They provide you with miniscule quality bonuses
- They have an insane penalty to reinforcement cost, which would make them a detriment to your nation even if they didn't cost military power
Here's a few suggestions on how this could be improved:
- Reduce cost of raising janissaries to 2 military power.
- Increase janis’ fire damage taken bonus to -20%; add a +50% reinforcement speed modifier
This would make janissaries an actually somewhat worthwhile unit, and would allow using them as a frontline meatshield, meant specifically to take on the first blow of the opposing army.
While some points were already mentioned (i.e. that the existing boni are actual pretty neat). You should also consider some additional points.
1.
Special units in general are an important flavor option to spice things up. In this case Janissaries can definitively give you an advantage in battle, even against somewhat equal foes, but this should also have an considerable downside since the Ottomans are one of the most powerful nations anyway. Plus you can avoid this disaster by not having too much Janissaries, considering their costs early on something that shouldn't be too hard.
I also should mention, that you have a huge advantage as the Ottomans by not only having enough money to buy high tier advisors early on (you can buy 3 Regiments for a year with an tier 3 advisor which is good even if it doesn't sound like it), but also the advantages the Ottoman Government gives you in terms of rulers (at least as far I understand it you should get better rulers by it, therefor mitigating mana cost further)
2.
If you compare them to other special units in the game, I'd say they are one of the better ones. Banners may give you discipline but also raise corruption, something the nations that can field will probably struggle with anyway. Additionally they take far longer to reinforce, which is a huge downside.
Rajput's are probably still worse event if their estate privilege is granted and you take their government reform plus you can only recruit a fairly low number of them, almost none until mid game (though maybe I didn't do that properly). Second point also goes for Cossacks who are weaker anyway.
Now the only ones left are Revolutionary Guards and Streltsy.
Rev. Guards are not only a late game unit, but also everyone can get them (and probably should if it's about fighting power only). Therefor I think it's no good to compare those two.
Streltsy on the other hand are arguably overpowered, no stat wise but because of their way of recruitment (+combat bonus). Trying to get any other unit to compare to them is just breaking the game further, something we don't really need I'd say.
Well and I guess I don't really have to mention marines that are actually weaker in combat itself (though I want to anyway
).
3.
As I don't know how experienced you are, I'd like to suggest to watch some videos about the fields you struggle with, i.e. if you have trouble winning battles even with Janissaries you should look further into the combat mechanic and actual army management (both in terms of composition and in how and when to move them etc.). Also different boni do well with other ones, I'm not that good in this my self but if I remember correctly damage reduction should pair well with morale boots since you can stay longer in battle anyway while grinding enemy morale fast (therefor ending the battle faster than by winning through casualties).
4.
This part is my personal opinion so please only look at it in that way.
EU IV is a game where things should be difficult. If there are actions that give you a pure advantage while there are no real downsides (like Streltsy recruitment) there is something wrong. Boni that are nation or culture specific should only be minimal to not only further harm game balance but also historicity.
The boni you want to see for Janissaries are insanely powerful if you compare them to the ones you can get at all. 20% fire damage take is not only double the amount you can get through normal means, but also in a normal game only the Ottomans can get them while still having the ones anyone can have. This would put the unit itself at least on the level most nations can get at all (just think about how strong that would be). Decreasing the recruitment cost is even worse - just imagine the Ottomans could field 24 regiments of this kind after two years of peace by only hiring a tier 2 mil advisor, that have no downside to them until fighting starts and it did it's only something that the Ottomans can easily take on (seriously double the reinforcement cost is negligible for them). Nobody would be able to stop them, I mean that's the case in SP anyway but in a MP this would require all other nations to team up on the Ottomans just to prevent them steamrolling everyone early in the game (the janissary disaster can't happen during the first 100 years of the game anyway so you can field as many as you like).
Instead those types of units should make up only the core of your army. They are elite regiments after all and any army comprised only out of such regiments who be... strange to say at least.