A certain contractor in my area has been asked (by several architects) not to bid on projects here. They've done 2 of my projects with exactly this strategy. Months of delays, massive extra work for the entire design team, just to get them to do what the plans say.
So what is a consultant worth in those cases?
Depends on your construction manager and how bulletproof your contract documents are. Even then, usually the only leverage the CM has is to withhold payment or pull the contractor's bond. Withholding the contractor's bond happens only ever in a blue moon it seems though and withholding payment can get sticky quickly, especially if it's "Pay when paid." Under "Pay when paid", nobody gets their final payment until the project is complete, and if the project is otherwise complete except for one trade, the CM would rather get paid and get the other trades paid and never see the work completed than drag out the payment process for months until the contractor finishes the work.
If they finish the work.
I know my particular state is stickier yet for state projects. Not only can completion for one trade hold up payment for all trades, but then all trades have their state contractors' and A&E points withheld. If you have too many points unavailable, you can't bid on new projects. So one contractor at the end can make it so an architect can't pursue new State projects until that project gets closed out.
So at the end when the contractor starts complaining about how rigorous the testing/commissioning/documentation processes are, the project team has financial incentive to absolve the contractor of those requirements for the sake of getting the project closed.
There's no substitute for getting a qualified contractor the first time 'round. No spec or consultant can guarantee the success of a project.
I've heard more than a couple times someone say "If you've got a good contractor, you don't need a contract. And if you have a bad contractor, your contract won't do you any good."
EDIT:
I should add -- not all projects are this bleak. In fact, most go off with only minor, to-be-expected complications along the way. I'm just trying to shine light on that the construction process has a lot of moving parts that can affect the outcome of the project. I'm absolutely serious when I say that the more hands-on the customer is, the better. There are a number of different firms and people looking out for the success of any given project, no one is in a better position than the customer to keep an eye out for their own short and long-term interests.