While HIIT may well be the best workout for burning calorie/fat (it’s not that simple but I won’t bore everyone with the details), it’s not HIIT if target power cannot be achieved. It’s not a simple matter of 8 x 20s max / 10s recovery - if a rider is unable (for whatever reason) to reach the required power, then it is not going to achieve the desired training effect.
As for steady efforts, you will see HR rising slightly - known as cardiac drift - even if you keep power as an absolute constant. If you are indeed training by HR, you may need to be mindful of power needing to drop during long efforts.