Hypo (SH) is actually an alkaline cleaner and disinfectant to boot...YEHAA!
How hypo works on surfaces it starts to neutralise on contact of grime within the dwell time. That is why sometimes needs to be reapplied and is fully neutralised within 30-60 mins depending what strength of mix you use. The reason why it's washed off a surface is not because it will do any damage in most cases but to simply wash the grim away so leaving the surface clean.
Used responsibly i.e. right method used to apply and at the right ratio mix for different surfaces/substrates sh based products have proven to be safe and cost effective way of cleaning a variety of outdoor hard surfaces including render, cladding etc on buildings for many years.
Kev if SH was so dangerous the likes of Benz would not be approved by HSE now would they?
Even smartseals new products contain SH and many other supplier brands too... go figure
Can I ask what's in your so say new wet&forget product?