It's a close call most of the time. If someone posts an obviously SU question on SF, then it will more than likely be closed. Even borderline questions get closed often. I almost always vote to close, rather than migrate.
Reason 1: I don't know SU well enough to know what kinds of questions are and are not duplicates. Migrating a question to a site that then has to immediately be closed as a dupe is not polite, usually.
Reason 2: The onus is on the OP to know where to ask their question. Certainly it's not expected to know each and every niche site that is in the SE network, however the m…