Roost is very common on Sigilyph (68.371), and knock only really cripples one set (Flame Orb CM, even though other sets would love their items). Knock doesn't do much of anything to Colbur CM Meloetta as well, and you have to guess right between Colbur CM/subCM (and specs) to know if Knock/Toxic are favorable moves, which generally puts Alomomola in a less than stellar position. A wrong guess with puts you in an incredibly bad position as well - and you're likely playing a guessing game for several turns - while a wrong guess for the Meloetta is less bad for it, because it still applies offensive pressure. When dropping a move for Knock, you're also dropping the utility that moves offers, which is itself bad for Alomomola. Without scald, you don't really threaten Venusaur, and so on.
I also think that Alomomola's ability to wall stuff is hugely overrated, as both sets leave massive holes in coverage. And it doesn't wall SD Gallade, which is probably the set you should be considered if you're using it, struggles with Scrafty (and outright loses to the terrible shed skin drain punch sets) and needs low rolls or godlike predict to switch into standard life orb Medicham - all assuming you're physically defensive. At the same time, that Alomomola has a chance to sort of beat 2/3 of these sets, despite them being massively powerful wallbreakers, says a lot. But you should just be using wish and getting out of there against any of those three, and many others, which isn't hard walling at all.
I also think that Alomomola's ability to wall stuff is hugely overrated, as both sets leave massive holes in coverage. And it doesn't wall SD Gallade, which is probably the set you should be considered if you're using it, struggles with Scrafty (and outright loses to the terrible shed skin drain punch sets) and needs low rolls or godlike predict to switch into standard life orb Medicham - all assuming you're physically defensive. At the same time, that Alomomola has a chance to sort of beat 2/3 of these sets, despite them being massively powerful wallbreakers, says a lot. But you should just be using wish and getting out of there against any of those three, and many others, which isn't hard walling at all.