Using this add-on, you can recruit not only standard multiplayer units, but also great units from campaigns: like Chocobone and Dwarvish Scout! Upgrade paths have been improved as well: e.g. you can level up Troll Whelp to Troll Shaman. And, in addition to six default factions, instead of Khalifate there is a creepy Monsters faction - containing the monsters from default/mainline unit tree!
There are 6 DC Eras: Default, Age of Heroes, Unlocked, Default + Monsters, Age of Heroes + Monsters, Unlocked + Monsters. Difference between Age of Heroes and Unlocked: in Unlocked, you could recruit ANY unit of your faction; that includes lvl 3 - lvl 5 units. In Age of Heroes, Monster faction is Unlocked as well, because there is a much smaller choice of units - half of them are sea creatures...
To preserve the balance, a few parameters of some units have been wisely changed, following the guidelines from other eras when possible. INFORMATION about all the changes (+links to Wesnoth unit trees) is available in a Feedback thread! Examples: 1) melee attack of Elvish Lord decreased from 8-4 to 6-4, so that Elvish Captain and Elvish Hero would be competitive against this unit. 2) While making Cuttlefish as an upgrade path of Tentacle, I checked Inky's Quest campaign to learn how much experience should it need