The ad-reward in the mobile app similarly applies in full percentage all the time, just like supercharging. Don't know why, to be honest, but why complain. So, all the "usual" army camp production boosts apply additive, but then Supercharging is a simple 20x and the reward for watching an ad is another simple 20% (or 30% with the reward for the hardened level) on top, no matter the underlying boosts. So, if all the other boosts combine to an overall army production from one camp of say 20k, and you have both a supercharge and an ad-reward on this army camp, your camp will produce 480k (or 520k) (20k x 20 x 1.2 (or 1.3)) for the duration of both boosts applying.
Good to know, if I get Army Camp Boost I won't focus on pumping it too much compared to others.
I wouldn't generalize this way. Because you are correct that you have diminishing returns when you have several boosts for one income and they combine to your disadvantage (sort of), but there is a lot that is calculated based on army income, so, you really shouldn't skip boosting your army income JUST BECAUSE the effects aren't that great. The number of armies that you draft is dependent on your army income. The amount of FC armies depends on your army income. The size of clan requests depends on army income. And perhaps I might even have missed one thing here. So, having a higher army camp production is never a bad thing. And will always account for more than just x armies more per second.
Edited 7/8/2022 23:04:58