I think that little blaster works at around 5000psi, they are a very useful tool but you need a good and patient operator....the water can do as much damage as any ecavator bucket in the wrong hands....many operators have years of experience carving their ay through roots and assume thats what you are paying them to do, rather than merely exposing the roots for clean cutting, its slower and messier than what they are used to. When you find a good operator who cares about trees hang onto them!!!
Protecting the trunk is occaisionally necessary but really as a last resort, since if there is a real risk of impcting the trunk what is happening to the roots!
Always be aware of the greater clearance heights construction plant requires too!!