Orb of Unity can be a very powerful card if used correctly, but can also be very vulnerable if not protected. It allows you to put a bunch of allies of the same type (for example, ants) on the field, and then cause them to become extremely powerful (high HP/AP) by playing this card. For each other ally of the same type on the field, allies of the same type gain 1HP and 1AP. The effect is global (it effects both your cards and your opponent's), so your opponent may get a little benefit from this card as well. It is easy to shut down if your opponent has cards that remove objects, such as Fresh Force.
Strong against: Decks that are focused on several powerful allies, in which case your opponent would not be able to defend against a large amount of ants with only several cards of their own.
Weak against: Object removal decks and heavy ally removal cards like Killer Wave, which can drastically damage your onslaught of ants.