Oneiros is a great unit. Its skill, which can be activated passively or actively, allows you to simply point at an opponent and block all damage that unit can do to your allies. This gives you very strong control over the damage enemies deal, and is very effective against units that deal AoE damage. What's more, it can be activated automatically against units that deal damage out of turn.
It is the only unit in the game that has this ability. It is also easy to build as a damage-denying tank, as it does not require speed cosmos, although it has a slightly high energy consumption to deal damage. Unfortunately, it does not gain immunity, which can leave it vulnerable to units that deal a lot of control.
Oneiros is an excellent control tank that can also eliminate an entire rival lineup. It is highly recommended to build, as it is one of the best units in the game.