SAM will only miss a Zantetsuken instant kill if they mistime their Chiten, which is user error, or if they get CC'd and focused down, which could be said for any other job. As long as you don't engage with and telegraph the clear intent of baiting out a Zantetsuken kill, it's fairly reliable. In any case, the instant kill is balls, the ability to go through guard can be retained, and the LB can just enjoy a damage buff without instantly dooming someone the moment they tap you.