RE: injury, I agree... for running. People new to running should start with slowly building up daily mileage and maintain a low-to-moderate pace. HIIT running is a high injury risk. But for cycling, go for it. On an indoor trainer, HIIT cycling is very safe and beneficial. Recovery is still important, so you'll see most HIIT programs go 3 days per week with light activity on the alternating days.
The problem with these HIIT recommendations is that they still come across as tricks to minimize exercise... as if exercise is something to avoid. People need to get out of that mentality. Do something active every day. Go a little harder every other day. You'll improve. Just be consistent and make it a priority in your life.