There are union contractors who go under and there are non-union contractors who go under.
What matters is the quality of the work.
I always go back to the same example. Say you have ten workers: five of them are unionized and five aren't. At the end of the day, you'll have five good workers, regardless. What's important is to check the quality of the work, given that the cost goes up every time.
When we receive a plan before working on a construction site, I throw it in my truck. It's merely a reference plan. After the first day, it won't be any good; it won't be completed. So subcontractors are asked to do the verification and to make sure it works, because it's very common for the wiring done by an electrician to conflict with the piping installed by the plumber. Professionals do those jobs, whether they are unionized or not, but without someone to check the quality of the work, you won't get the result you want.
Whether or not you belong to a union has nothing to do with the quality of the work done. But it does affect the cost. It's totally false to say that a non-unionized worker won't do as good of a job.