Just like with most things, there are some caveats. The training paradigm used in some of these workouts values completion of the workout above all else. Mix that with some healthy competition and add a dash of peer pressure from the instructor or your friends cheering you on and you've got an atmosphere where it is "go hard or go home." Minus the actual fighting, it's sort of like a real life version of Fight Club in your local gym or living room. Imagine being peer pressured to see if you can best your previous workout record while ignoring your body's physical limitations. It's exciting and it will push you, but is it safe? In this push to work past physical boundaries and ignore fatigue, improper form and poor movement execution can occur. This is sometimes necessary on the athletic field or in other unique situations (ex: being chased by a pack of feral dogs). Unfortunately, it also opens the door to significant musculoskeletal injury, such as sprains, strains, overuse injuries, or traumatic injuries such as tears, ruptures, or fractures.
HIIT can also cause some other serious problems. For example, cardiovascular issues, overtraining, stress incontinence, and rhabdomyolysis (among others) have also been shown to occur with improperly supervised or performed HIIT. Rhabdo is particularly alarming – it can result in dead muscle cells flooding the bloodstream and overwhelming kidney function causing a massive shutdown and even death. While the above injuries are not common, I do see a constant stream of injuries from these types of HIIT programs at my clinic – HIIT aficionados are quickly becoming even more ubiquitous than runners in my clinic. While I like the job security, I don't like people getting hurt. Fortunately, other than some wrist fractures and some shoulder dislocations, I have yet to work with a significant number of people who were seriously injured doing these HIIT routines beyond the normal overuse injuries.
One of my PT mentors often advises me to think simpler when confronted with patient exercise progression. His advice is sound and essentially boils down to the following statement; master the basics. Then, and only then, is it safe and appropriate to move on to the next step. Due to its high level of multi-joint, three dimensional, and plyometric movements that can be coupled with heavy force production, HIIT can involve significant complexity. It should safely be progressed into as part of a general fitness routine once exercise-appropriate basic skills are mastered.