Design bug? "Supported recurring units" for Membership contribution. Why?
The big answer is that recurring contribution features were built with regular donations, not memberships, in mind. That memberships work is a nice bonus, but I think it would take a bit of investment to rework that the way you describe.  With most memberships being more standard durations, nobidy has stepped up to build that.

In your specific case, many payment processors won't even support biannual recurring payments--their maximum interval is one year. A potential reason for this is the proportion of cards that would expire between each renewal.