It would be useful to be able to check if an item is a member of a collection, when filtering in a formula. Right now the best workaround seems to be to convert the collection to a concatenated string elsewhere (since you can’t call join inside a filter), and check for regex string match on name. This would not be robust to names containing other names, and is more roundabout than it would be if you could check for collection membership.
Somewhat related to this as well, for ways to use collections in formulas: "Union" function to merge multiple collections into a single collections field