Root - Worst match-ups in the game
Root is a hard game to design, and adding a new factions is literally exponentially more difficult because you have to design and balance it around more and more factions. Still I would consider it a marvel in design and is currently my favorite game, but I have to admit that even within "reach" (so theoretically compatible factions) there are faction combinations that shifts the balance on one side more than others. Some factions are significantly worse if a another faction is in play, and here are those I think are the most significant examples from my experiences (assuming 4 players, because I can see how it could change drastically at other player counts):
Vagabond vs Eyrie: Vagabond is considered by many top 2 in the game for many reasons but a big one is that slowing him down by attacking him grants no points, so everyone hope someone else will do it, which allows the Vagabond to run away with his game plan unhindered. The Eyrie is the exception, because he loves having a target he can fight multiple time at low risk, and be able to comfortably put more cards in Battle than he would otherwise. Just by having the Eyrie in the game, a Vagabond has to stay away from him, and that alone is enough to slow him down by a full turn or two. LotH can also hit him easily, but his combat targets are much more precious and he actually doesn't want to go towards the Vagabond because he needs empty clearings and he cannot make the Vagabond leave during his turn.
Riverfolk vs Woodland Alliance: Every faction need cards, but when the price is a worker and you have so few that a single warrior is worth more than a card, you don't buy any. Especially since the merchant is absolutely not obligated to give it back, ever, and he won't. Anyway he needs you to rule to be able to spend them, and that is simply not in your game plan to rule almost anywhere. As a result, the Woodland Alliance will never buy anything from the Riverfolk, which is their one weakness. Riverfolk is my favorite faction, but I really am less tempted to play them is WA is in play.
Woodland Alliance vs Lizard Cult: This is the big one, Woodland alliance rely on having a low board presence (none actually), and build up until they can build a building, which essentially starts their second form. And this point, they try to finish the race while guarding their building intact. Well the Lizard can just steal it, and they get a soft "turmoil" because of it and against good players is very hard to recover from. It costs the lizard almost nothing, and all they need is to have the hated faction to be the right clearings. Knowing that, them and every other players will make sure it happens maybe the exact round the WA managed to build it. And there is nothing he can do about it unless he somehow can manipulate the discard pile toi save himself, every single round. This is the only one that I actually think is an impossible matchup against competent players.
Honorable mention to Duchy vs Lizard Cult for the same reasons as above, but hurts so much less than it does for WA.
And last one, Keeper of Iron vs My brain: Only faction I never won with, and it is not because they are bad, it is because for some reason I cannot wrap my head around optimizing their game plan. It feels sooo much more difficult to play than any other faction, to me at least.