Soft Washing is relatively new in the UK. You can Hypo or Bio.
What are the differences?
Bio is bio degradable does the same thing as hypo but takes longer and does not use bleach as a source chemical, but takes weeks to react and longer to get your cheque for the job. ( some tell customers it's like weed killing, manage the customers expectations by stating all will be good in a few weeks, then take payment or some wait and come back a few weeks and collect payment after the result is shown)
Hypochloride, can give burns for those not trained, can harm plants or damage some surfaces. Training is essential. This chemical is under your kitchen sink ( bleach/toilet cleaner) so not the toxic menace made out, in fact treat this like you would the stuff under the sink, you know...dont drink it, splash it about, get it in your eyes/skin..... hence training essential.
Hypo is instant, collect the money on the day, no convincing required.
Concerns? Yes those with employees.
Infact I would have the same concerns if my staff were cleaning windows at 10 storeys. Thats why we have IRATA and IPAF
Conclusion?
Your decision - bio or hypo will be either health and safety concerns or environmental concerns. Training is key as with everything.