Unit used in the term of calling someone something can be used as an insult or just for the hell of it. I depends who your calling Unit, and what they have done to deserve the title. Friends calling friends Unit is not considered an insult, but if you say it in a mean way then it is considered an insult.
A girl and her friends are hanging out somewhere and the girl does something silly, her friend say "Your such a Unit!". This is used in a friendly way.
Seeing someone down the street that you dislike and saying "UNIT!" is said in a insulting way, and is therefore not a compliment.
Writing about somebody on a distribution list, forum, discussion thread, etc in a derogatory, ridiculing way in an attempt to get the person to respond merely to see if they are still alive and reading the list. Originated when the author of this article mis-spelled speaking when trying to get a rise out of a distribution group member.