I think there's two aspects of building-based promotions that should be remembered:
First, as a warmonger you also get this benefit. It's still a promotion you don't need to take.
Second, as a warmonger you probably want to build these military buildings for their own merits. Everyone else wants a minimal number of these buildings. Even if they do have access to boosted units, they have a more limited production pipeline to make them. This might only matter if attrition becomes more a part of the meta, but maybe that should be the case?
We could also change Authority to shoulder some of the weight of making warmonger units gain more of these benefits. Things like yields on unit-production, direct boosts to barracks line buildings for warmonger-only interests (unit production boosts, maybe?), or preventing gold-purchased units from benefitting from building-promotions and giving Authority a policy that counter-acts that.
First, as a warmonger you also get this benefit. It's still a promotion you don't need to take.
Second, as a warmonger you probably want to build these military buildings for their own merits. Everyone else wants a minimal number of these buildings. Even if they do have access to boosted units, they have a more limited production pipeline to make them. This might only matter if attrition becomes more a part of the meta, but maybe that should be the case?
We could also change Authority to shoulder some of the weight of making warmonger units gain more of these benefits. Things like yields on unit-production, direct boosts to barracks line buildings for warmonger-only interests (unit production boosts, maybe?), or preventing gold-purchased units from benefitting from building-promotions and giving Authority a policy that counter-acts that.