i used to be a roofer , my main concern would not be falling but damage caused py using high pressure as mentioned in the previous reply, you said target roofs over 25 years old but from experience that could be expensive, clay tiles are extremley fragile at that age as are some concretes, as for slate stay well away.
Also if you were cleaning a hip and broke a bonnet tile half way up how would you replace it without disturbing the mortar on the other bonnets? you could end up paying for a roofing firm to come and relay the whole hip!
Also be wary in valleys, around dormers roof lights, and chimmneys as these are all liable to let in water in driving rain let alone using high pressure.
Matt