There appears to be a bit of a dispute going on in the FAQ answer here. On one hand, a group mostly consisting of Stack Overflow moderators says that if it isn't immediately clear that an answer isn't an answer just by briefly glancing at it, the standard "not an answer" flag shouldn't be raised and a custom moderator flag should be raised instead, clearly explaining why the answer isn't an answer. On the other hand, another group of users say that the standard flag should be raised on all non-answers, regardless of whether they're blatant or not.
I understand where the SO moderators are coming from, because they don't see the question in context or comments when reviewing flags, and they get a lot of flags every day that they need to handle quickly. Thus, it's entirely understandable that they would prefer that the flags they handle contain a clear description of why the answer...isn't.
However, using custom flags places more workload on moderators and bypasses the community review process. The standard "not an answer" flag also places the answer into the Low Quality Posts review queue, where community members review the flagged answers. Additionally, community members reviewing are shown more context than moderators reviewing in their queue. I can think of some other reasons why raising the standard flag even in non-clear-cut cases is better:
The standard flag goes immediately to the community "Low Quality Posts" queue, while it is kept out of the moderator queue for a period of time (15 minutes on most sites, 60 minutes on Stack Overflow, no delay on meta sites). This gives time for the community to handle these flags and reduces workload for moderators.
If someone casts a standard flag while someone else casts a custom flag, and the answer is deleted in review before a moderator gets around to handling the custom flag, the custom flag isn't dismissed automatically (for good reason) and a moderator still needs to process it.
Based on this, if an answer is a non-answer, but doesn't look like a non-answer at first glance, should I still raise a standard "not an answer" flag, or should I raise a custom flag? An answer from the team would be very nice, though opinions from other users (especially moderators on other sites) are cool too.