What sort of projects are those? Not software I assume? I've yet to work on a project in my life which remained in the initial scope. Plus debugging seemingly trivial bugs can take entire work days.
To avoid major bugs, I make it clear up front that if they stick with the stack that I choose for them (which is a stack and development pattern I've been using for the past 4+ years), the product will be delivered quicker versus if they need to use a specific stack.
Typically the client doesn't care how the product is built and generally doesn't care how it looks as long as it's easy to use and does the job they need. Everything is outlined in the bid/contract so there are no hidden surprises about the work being done.