Exercises that require a lot of technique, bracing, or are heavily loaded should be avoided with this method. The key to a good set of 21s is not having to stop during the set. Certain implements like barbells are harder to employ 21s with because your ability to brace or control the barbell may limit your ability to take your desired muscle to failure. If your core gives out before your arms or legs, you might end up spinning your tires and even risking injury.