I guess my biggest concern with SFCs would be resistance to rust, longevity and fit. While I agree with you somewhat on the flyswatter analogy, cheaply made parts have a tendency to fit poorly. Also, a cheap grade steel with a weak coating (or none at all) is going to rust out and possibly fail over the years. One thing I noticed is some SFCs may not be sealed at both ends, or have cupped bends, which could cause them to collect or pool moisture and accelerate corrosion.
Of course, half the battle is having a good welder do a good job putting them on. I paid 200$ to install my MM SFCs, and I was less than thrilled with the speed and quality of the shop that I went with.