I tried this app and could barely move after. The workouts are intense. A whole different level of intense! I love how I feel when I'm done. You all should try it!
I think it's good to have another currently popular way to get people think about and work on their fitness. BUT: from what I understood is that Freelethics is all about HIIT.
Problem 1: HIIT is not a good start for those not experienced with getting their body move. You should first do your homework and then gradually push further to keep challenging your muscles. Example: when learn driving you won't start at a race course.
Problem 2: The program is supposed to be about always getting faster and doing more reps. Thus, the precision declines. When working out you should always keep an eye on proper movement. It's not only the efficiency that improves, it also prevents you from injury. You don't have to rip 100 pushups per minute to improve your strength.
Thus, I'd suggest to learn your exercises by heart, start slow, aim for clean movement (best if observed by a professional trainer) and then after some weeks maybe set precise shocks by HIIT. But you can be happy and fit without it. It's just a matter of efficiency. Me personally I enjoy working out - why cut time to the minimum? :-)