The general consensus of the internet is that the Deck of Many Things is overpowered and unbalanced. I can't see why. Sure it can make your character a king, but it can also just make your character a vegetable. That seems balanced, just extreme. The only thing I can see is if a DM gives it at the wrong time and it derails the campaign, but that's not really the items fault.
It has nothing to do with balance.
The problem with the deck of many things has absolutely nothing to do with “balance.” It’s not even supposed to be balanced, nor does anyone particularly expect it to be.
The problem with the deck of many things is that there are way, way too many cards in the deck that simply ruin campaigns. Draw the wrong cards—and if you draw more than a couple, you almost-certainly will—and it becomes impossible to figure out how on earth the campaign is supposed to continue from there. It’s a nightmare to DM for, which makes it a nightmare for players too—because they rely on the DM being able to, ya know, DM. Literally any introduction of the deck of many things into a campaign is just begging for terrible problems—which means unless you are looking to draw your campaign to a “zany” conclusion, it’s a mistake to put it in the game.
Balance is more than just an equal probability of good and bad
While one could argue the net effect of the deck is neutral (or more likely, negative), that doesn't change that the deck is overly powerful in either a positive or negative direction per use relative to typical power levels provided by the standard game mechanics.
Imagine a simpler item that, when used, has a 50% chance of instantly killing an enemy, and a 50% chance of instantly killing you, no saves. While it's technically balanced in terms of all possible outcomes (you die, or they die), each individual outcome by itself is immensely powerful (either they instantly die, or you instantly die, not both). Both outcomes are extremely powerful, and no other game mechanic offers power like that, so it would be considered to be overpowered.
Similar to the deck, while good or bad may come, the good has a chance to be overwhelmingly good, and the bad overwhelmingly bad relative to normal game mechanics. There's a risk, but the outcomes themselves lacks balance, and thus can easily break the game.