Answers in comments are a thing comments aren't for. It's an issue enough for RPG.SE and for IPS.SE that we have meta topics saying not to do it, and on the site I moderate, "answer in comments" or some variation thereof is the single most common use of custom moderator flags on comments.
Despite this people often don't recognise it's something they ought not do, and there's lots of sites where answers in comments are in abundance and not cleaned up.
I suggest the addition of an "answer in comments" flag — to prompt people to use it, to recognise the behaviour's definitely officially something to not do, to assist with speedy removal, and to help formally count "answer in comments" as a flag reason such that moderators might be prompted to contact those users to advise them they're using site features improperly.