I'm not familiar with Ninja's, and never had an inline heater. But, with previous machines with in-tank heaters, the elements would gradually scale up and needed treatment. However, when I started to use an acidic rinse more often (this was a LONG time ago) the scale never had a chance to build up. When I started using products which required only a fresh water rinse, I used an acidic rinse once a week to control the scale.
Is this practice viable with an inline heater? Obviously, if a detergent rinse accelerates the scale build up, an acidic rinse would need to be used more often, but with good cleaning practices, the quality of the work would not be compromised, it maybe even enhanced.
Safe and happy cleaning
Ken