Didn't the clients expect holy 8 hours in this case?
Another class of freelancers/consultants which are missed out from this discussion (hourly rate vs other): long-term/indefinite flex-time workers. No estimations or promises, just pay-as-you-go for several years or more. With the hourly rate, in this setup, one can work 5 hours one week, 0 hours second week, 35 another week... So a kind of very flexible employment arrangement disguised as consulting.