At secondary school I had two teachers of Physics who didn't have physics degrees, nor teacher qualifications.
The first was an okay teacher, not inspiring, but not totally off putting either. I think his qualifications were in something related like electronics, but he'd been teaching for several years when I had him. He got things wrong though, sometimes dramatically wrong, and he wasn't brilliant at being questioned around the subject because he wasn't that knowledgeable.
The second teacher had been involved in research for something like British Rail, looking to improve signals etc. I had him in his first year of teaching, having worked for them for about 20 years before. I think he had a degree in engineering, but I might be misremembering. He was an excellent teacher, knew his subject inside out (or at any rate had researched it well before teaching us) and was very good at engaging a class on the whole range of things we had to study.
So the problem is with underqualified teachers, is that, just like teachers, you get good, okay and bad ones... as in any other profession.