On the flip side, most projects I work on on the side are now are bid based vs hourly based. I wasn't a fan of it at first, but now I've really come to like it.
I know what I'll get paid, and the client knows what they're going to end up paying for the work that's been scoped out. It's up to me to determine whether I'm managing my time/rate correctly to make it a value for the client and that I'm getting what I feel I'm worth.
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.
It's software development. It's true that the scope is flexible, but it's not to a point that I think I'm doing an uneven amount of work compared to what I'm being paid, and if the additional features are too big then we begin working on the scope for another bid.
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.