Winterizing tankless water heater

Yeah, the guy doing the walk through seemed excited that the tankless was a new thing, but I'm learning more about it and thinking it's an overly complex and expensive thing that could be a problem child. I found a Youtube on winterizing one, but he was using air. I'm just not convinced that works, especially when the manual indicates it doesn't hold enough water to cause a problem. Key thing: It still holds water after blowing it out, and there's another Youtube of somebody showing how he repaired his after it cracked from freezing despite blowing it out.