I've been using the v0 API extensively in my installation/deployment scripts for a year, and only once bumped into an issue where something had changed names (database related, if I recall correctly). Only lately it became a problem with the complete overhaul.
But yes, v0 has been an undocumented API and I had to reverse engineer the whole thing, using the javascript code from the old dashboard. Still, it was "stable", as sean said.