But if the hearing instructor is a poor ASL instructor, such as not teaching voice off, etc, then that's more harm than good.
Would you want to learn to drive a car from someone who teaches that it's ok to run red lights, tailgate, etc? And then find yourself getting tickets or into accidents because that's what your instructor taught you?
I don't think what you are comparing really fit together. There is not the danger level in making signing errors that there is from the driving instruction you have described.
Where I live a hearing instruction is the only choice that I know of. I took such a course two different semesters through a community college. I made no contacts to continue using it. But did get enough that when I went to denomination wide church event that had someone signing it did help some. That was many years ago and I have since forgotten 99% of what I learned from not getting to use it.