Anyone working at your property, be they taking a chimney down or putting scaffolding up, should be insured to cover any damage they may make. And that cover is for the worst case - eg the chimney or scaffolding falls through the roof, hits the gas or electrics and burns the house down.
No firm asking you to disclaim their potential damage and limit your rights to claim against them should be trusted.
Ask to see their insurance certificate.
Also be clear who is working for who. Are the scaffolders working for you, or working for the building firm? You need to know as that determines who is responsible.
Do not sign any disclaimer. You not only potentially sign away all your rights to claim for damage, but also potentially invalidate your home insurance cover too - so may not be able to claim on that if the need arises.
For reference, all tube scaffolding should be designed to a standard called TG20:21, and not just made up. So for anything "none standard" such as using a balcony, they would design the scaffolding with special software. Once erected, there should be a formal hand-over certificate to prove the scaffolding was properly designed and erected.