Mercenary Camps are a Tavern replacement available to Germans by sending a card of the same name. No matter how much they used mercenaries, replacing the Tavern of the most beer loving civilization in the game is a very poor thematic fit.
Mercenary Camps should not replace Taverns, they should be an additional building that is constructed by the Explorer (with a build limit). This would also be more fair than replacing existing Taverns since Mercenary Camps have half the cost and stats. Furthermore, they should be for production only, and not provide any trickles. Changing the model to remove the hospital operating table would help make them look better and more distinctive too.
Mercenary Camps should also be available through more avenues than just a single German card. The Mercenary Contractor politician could enable Explorers to build them. There are also several Swedish and Dutch cards that enable mercenaries in regular production buildings that would be better implemented by instead enabling the mercenaries in a limited number of Mercenary Camps. This avoids a lot of the issues with giving them unlimited access to unit types normally not in their roster, and would enhance the strategic decision making since they would require a separate production building.
In addition to improving mercenary stats and enabling civ specific mercenaries, this politician now also enables the Explorer to build a limited number of Mercenary Camps.
Instead of converting Taverns to Mercenary Camps, it should simply allow the Explorer to build a limited number of them.
Buffs and enables additional mercenaries at the expense of your regular troops and allows Explorers and Envoys to construct a limited number of Mercenary Camps. No longer enables the training of mercenaries at the Barracks and Stable.
Buffs and enables additional mercenaries and allows Explorers and Envoys to construct a limited number of Mercenary Camps. No longer enables the training of mercenaries at the Barracks, Stable, or Galleon.
Enables the specified mercenary and allows the Explorer to build one additional Mercenary Camp.
There are also a number of other issues with the standard training of mercenaries. These could be addressed with a rework of Taverns and Monasteries, and by limiting the randomness of mercenary availability by organizing them into Mercenary Companies.