In any type of investment none of company will give you assured returns. If they make advertisement of assured return of any type of investment but in Disclaimer of the brochure or company letter head they may mention "subject to risk of market risk etc"
"Market risk. Market risk is the risk of losses on financial investments caused by adverse price movements. Examples of market risk are: changes in equity prices or commodity prices, interest rate moves or foreign exchange fluctuations."
So here you should take a confirm written from builder if such thing does not work as per your expectations after 5-10 years than "X" amount should be paid by Builder.