Fat loss is different from weight loss. Weight loss accounts for loss of everything, muscle and fat, which traditional cardio is infamous for (when over done). HIIT is notable for its "fat loss," the fat goes without the muscle leaving. it's the primed way of cardio for those trying to loss fat while hanging on to muscle. One down fall of HIIT, is the mental aspect. You don't see weight scale results as well so it messes with your head. Makes you wonder about the results you're getting, so use the mirror or actual Bf tests to determine your results.
HIIT 3-4 is great and 15-20 min is ideal, anything more then you might not being pushing hard enough. If you're doing HIIT, you're good on the muscle tip.
HIIT, Sprints in particular, create the same biochemical reactions within your body that occurs when you resistance train, it's all to tha good.