In terms of Utility and Cost Effeciency
This is one thing I noticed while playing the game, mercenaries just seem to defeat the purpose of building training structures and spending influence to unlock units. It just seems that because we can recruit mercs of basically every unit type, and the fact that they start at much higher levels, there doesn't seem to be a very productive reason to train them at your home city.
Training at your home city costs both gold and food or wood, and it takes some time for the unit to actually finish training. With mercenaries they only cost gold, train instantly, and can start levels as high as level 9, saving you the trouble of actually leveling them up.
Where as I enjoy the convenience of being able to start with a nice well trained army, I don't think it's right to outright void out the purpose of the training structures in the home city.
I believe that for the extra amount of work required to train units at the home city, we should get some kind of benefit, here is a number of suggestions.
1. Raise the mercenaries costs, but give discounts to players that have the particular unit unlocked for their home city, also give further discounts for each subsequent barracks that they have. Simply raising the costs of mercenaries won't make trained units more popular, because it's still more convenient to purchase high level units than to grind low level units up. Players will just end up having to grind for the gold more to obtain mercs and there would still be little use for the barracks. Also, number 1 would make building more than 1 type of barracks more useful, because would you really need 2-5 archery ranges when humans have limited building plots as it is.
2. Make trained units either stronger, or gain experience at a faster rate. Seeing as how they are trained from your home city, it isn't that they are better trained than mercenaries, I'm sure those mercenaries are pretty well trained too. But mercenaries fight for coin, where as your trained soldiers fight for your people, there should be some particular benefit to symbolize their more heroic zeal to protect their families.
3. In contrast to Number 2, make mercenaries not gain the benefits from unit upgrades, such as blacksmith upgrades. This way mercenaries serve as the early game questers to help new cities build up to their own military might. In a sense your paying them to fight for you, but you didn't offer them any of your training services or equipment, furthermore once again, they are fighting mostly for coin, perhaps when morale gets implemented mercenaries should have a lower morale in comparison to regularly trained units, as they would care more about their lives than protecting some city that hired them.