Each surface is very different and therefore each surface needs to be treated differently. One surface might be porous while the next surface could be very dense in nature. Algae, mold, and mildew may be growing on surfaces like: wood, brick, vinyl siding, stucco, stone, metal and roof shingles but each of them needs to be cleaned with a potentially different chemical/soap that will bring them back to there original beauty. One surface like brick may need a light acid wash with more pressure to get clean and the next surface like wood will need less pressure with a gentle wood cleaner so the wood fibers are not shredded to bits.
Thinking that high pressure removes everything is a big fat lie and will result in scarred wood to etched vinyl fences and composite decks. This is why it is so important to ask your “professional” pressure washing company what products they use and if they are harmful to use. Using high bleach solutions in soft washing to heavy stripping agents in wood cleaning can be effective but they can also damage plants, trees, and different surfaces. Make sure your pressure washing company also carries liability insurance so if an accident does occur they will be able to resolve the issue.