Danger One
A person on your roof can break tiles, especially clay tiles and especially small clay tiles which are very expensive to fix and you only find out later when internal water damage occurs.
Danger Two
Will a person pressure washing your roof tell you that they have broken a tile / caused damage?
Danger Three
Pressure washing can strip the top protective layer off the tile rendering your roof weaker and dramatically lowering the life of your roof.
Danger Four
A person on a wet slippery roof is a dangerous situation. Many accidents do happen. Does your home insurance cover this?
Danger Five
Pressure washing is very messy and the moss/debris can block your gutters and downpipes.
Danger Six
If your membrane under the tiles is weak or perished you have a very high risk of internal water damage.