You have to be good with headshots to be able to use the Ambassador. I'm decent with it. I usually use it while running after I've lost my cover with the Dead Ringer.
Ambassador does win, especially now, cause it gets full crits... not sure about this, but in Turbine, i think i use the ambassador more than my knife :3. I always sneak into the enemy's air vents uncloaked. God, 2 headshots and your dead, silly sniper