My builder uses Houzz Pro to plan the timeline and give detailed estimates and daily updates with photos of progress, and provide invoices at pre-agreed key points in the build. This made the whole experience much more efficient and stress-free.
A good question! The answer is two-fold. Indoors (as in this application) the answer is no, it is stable. Outdoors rain will cause some patina changes and possibly some run off. We would recommend using a sealer on the stone sills. Having said that here are some photos of a 6 year installation with minimal run off
CoorItalia