Lack of long-term randomized scientific studies proving the diet works and is safe. A randomized study distributes participants in a deliberately random way into either the non-tested diet group or the special diet group. Some fad diets state there is research to support their claims, but the research is only done with a few people or does not exist.
What does a HIIT workout look like? You could jog for two minutes, sprint for one minute, jog for two minutes, sprint for one minute. Or you could do a HIIT workout on a bike, or by running up stairs and then jogging back down. The key is that you go relatively all out for a short period of time, then recover by maintaining a moderate level of intensity, then go again.