The kind of HIIT I'm doing is only 10-20s bursts, to stay within ATP-PCr energy metabolism (little lactic acid production). A short sprint followed by 2 minutes rest. That's hard to achieve with a treadmill.

https://en.wikipedia.org/wiki/Bioenergetic_systems#ATP%E2%80...

I guess it depends on your age and current fitness level. When I was young and fit, I could outrun the treadmill for the periods of time I needed.

The treadmill also wasn't able to change speeds fast enough to really do the good Sprint simulations