There are, I believe, multiple dimensions to the problem.
1. Alps have very high initiative (on an account of no armor penalties). This means, practically speaking for mid game and late game companies, they always go first. This means that generally speaking your first turn is lost, because you’re too busy waking people up. There’s also not enough room in the formation to keep everybody spread out to limit the number of people slept in a large enough company.
2. They only appear at night, which is appropriate. The problem with this is that range brothers are practically speaking useless, because they won’t be able to hit Alps very frequently at night. Combined with the fact that Alps have a very, very short detection range on the world map, you won’t be able to swap out the range brothers for melee brothers in time before combat is joined.
3. The Alp’s action economy is very efficient; they can either sleep twice in one turn, or sleep and move. This is an annoyance to the player, because they’re already hobbled by having to wake people up, which means they’re only moving two spaces at most per turn. Typically one if you’re caught in a swamp. Because of this, very commonly the player has to spend around 20 minutes chasing Alps around the map by cornering them so they can’t run away, and then eventually kill them.
4. One obvious counter to Alps is the use of dogs to increase the number of targets that the Alps have to sleep (not to mention, you don’t actually care if the dog dies considering dogs are only 300-400 to replace for an armored dog). The design issue with this being that, considering Alp trophies are only basically used for the trinket, a few potions, and dyes, that makes Alp drops essentially worthless, because the trinket slot is being consumed for the dog. Not to mention that the hexe trinket has higher resolve amount than the alp trinket, while hexes are in general much easier to deal with (by virtue of them not having default range resistance). In short, the alp trinket does not really warrant specifically looking for it, and does not provide meaningful resistance to the Alp sleep, compared to adding another target that can lock the Alps down by getting them caught in melee.