We have a mix of 4 Unit granting methods:
1. Normal unit tree (all the shared units, warriors, longbowmen, etc). Units granted to all civs (in general). We probably own't be adding many units here, maybe a few ships, but this unit tree is pretty solid.
2. UU's. Units that replace units in the normal tech tree for a specific civ. The non-human civs will have tons of these. The human civs could have none.
3. Resource upgrades. Upgrades to existing units that are only available if a specific resource is available. This resource could be Fire mana, Nightmares, Hellstones, Iron, Banannas, anything. Most mana types will probably allow a few of these.
4. Summons. Units that can be gained through spells. In general this means they will only last for a turn, but this law can also be broken. Summons are always other-worldly units.