I've seen people mess up an Ambitious Card routine. There is no guarantee of success with any mentalism or magic trick. They all depend on the skill level, experience, and ability of the performer.
Speechless works for me every time as long as I follow the guidelines and conditions and select participants that meet my criteria. I can only speak for myself and my own experiences, but with anything it takes practice to master. Speechless is not a difficult technique to master but it does take confidence and showmanship to get the best results.