It's boring when half of the game units gets unused in an optimal way to play.
I agree, and it's something that has creeped into the mod.
The biggest offender right now are ranged units, and melee mounted units before tanks. Though tanks have their own issue.
Ranged units are ok around crossbows, but they are just too easy to kill for what they can do. Melee units backed with siege weapons just work way better. Siege weapons only downside is the slow movement they have in enemy territory, and they are also easy to kill if attacked, but so are ranged.
Melee mounted units used to be really good as their CP and quick movement allowed them to circle around units to attack behind melee walls. However, I find in practice you just end up trading your unit, to kill a single enemy unit, as the next turn you exposed unit get murdered. I think this changed because the AI is just better at unit movement and moving units in formation. In theory, particularly on defense, you can use your roads to do hit and run attacks, however ranged melee units just do this much better. Once you start getting into tanks they start to get somewhat better, but the oil cost limits how much you can really use them. Plus when they start to get REALLY good, as in modern tanks, you end up converting them a couple turns later into Giant Death Robots because the late game tech scaling issues.
My answer to this would be to remove the ZOC and terrain movement costs when attacking (if possible). This would allow better hit and run attacks if attacks always only cost 1 movement. I would allow mounted units to use enemy roads to make this more of a thing too. Also looking at the tech scaling issues in late game will allow actual tanks to have more of a use.
My answer to ranged would be to to first rework the first two promotions. Attacking 50%+ or 50%- health bonus needs to go. Instead we need to have a defense bonus (that gives a bonus when defending in forts and cities) and a general attack bonus in the other tree. This will allow someone to not only specialize their range units, it would also give them a niche they sort of lack now. If my memory serves me the original change to the promotions from a city or unit bonus was the AI is too dumb to understand to use their specialized units in a specialized way. I am not sure if this is still the case? I would also bake in a bonus to attacking units close to full health in ALL ranged units.