Well, the item is an artifact. It's not so much broken as it breaks the game.
Heres the lowdown, the more random things are in D&D, the less it favors the players and the more likely it is to cause problums. Thats why the rod of wonder is a poor item, why a greatsword is (usually) superior to a greataxe, and why enemies with large, random damage ranges are dangerous.
The deck of many things is about as random as it can get. Not only is it random, but effects can either completely ruin a character, or make them really overpowered in a short time. Most DMs who have used the deck of many things before learn that the only purpose of the deck is when you want to end the campaign, and you want it to end with a bang(perhaps literally), apart from that reason the item really has no good reason for existing, and should be avoided like pun pun or a rust monster.
As for the summoned creature thing, I'd say that since summoned creatures are an extension of their master, the deck would have the card effect the master. Thats purely an ad hoc ruling though.