Yggdrasil is a cooperative board game in which the players unite as Norse gods to defend against the mythological monsters marching on the titular world tree. You don't win the game so much as survive it; one card from a deck of enemies is revealed at the start of each player's turn, and you must stave off defeat until the deck is exhausted.
In the way of most co-op games, it's fiendishly difficult. (By which I mean it's challenging, not necessarily complex.) The system is stacked against you, and random chance can make any one playthrough particularly tough. This is sort of expected in a co-op game, and not really a mark against it. But the game does have issues that soured my experience.
First, it does not scale well for the number of players. To be fair, the rulebook does warn you that the game will be harder with more players. This is because an enemy card is revealed at the start of every turn. Each player can cultivate a narrow specialty against a particular kind of enemy, but they only get to apply that specialty when their turn comes around. If you're good at a thing that becomes a pressing concern during some other player's turn, the group may have to wait a while until you can "take care of it" -- and that's time you may not have. Yggdrasil claims it can be played with up to 6 players, but it felt to me like any more than 4 wasn't truly practical.
Second, there might be a bit too much randomness in the system; there certainly is for my taste, anyway. There's the shuffled deck of enemy cards that determines where the players must focus at what time. There are bags of chips (containing "hits" and "misses," loosely) from which the players must draw to accumulate strength for attacks. Then there's a die that's rolled both during combat resolution and to apply some of the game's effects. Any one or two of these sources of randomness feels pretty typical of the co-op genre, but all of them together makes for what feels to me like a too wide a variance in difficulty.
Third -- and most discouraging in my experience -- is the design of the characters each player assumes. Everybody takes a particular Norse god -- Thor, Odin, Freyja, and so forth -- each with its own unique ability. This is the backbone of a good co-op game, in my view: give each player their own way to affect the game as no one else can, and everyone is much more likely to feel they're contributing in some way to the group. Some characters in Yggdrasil have powers that can be disproportionately undermined by negative effects during the game. While some character abilities are applied quite generally, others are tied to specific game actions -- actions that can be turned off or rendered useless during play. Put another way, the game gives each player a "role" to play in the group... and then can proceed to make it impossible to pursue that role. That in turn can make a player feel useless to the group effort, or even a detriment.