That hits the nail on the head, Nick. I have some movies that have very minimal extras and extremely high quality. Every once in a while, I'll notice that if I'm backing up a DVD to DVD-R, some discs will require a surprising amount of compression to fit, even though they're well short of two hours and contain to extras.
Obviously you could have any disc released at the same technical level of an SB, but the problem is you'll never know unless you buy it. The SB label virtually guarentees a high level of technical quality, while another company might acheive a similarly high level but not make any note of it.
In short, I've never been disappointed with the picture or sound of any SB DVD, although of course I've bought them only to realize the movie itself wasn't as good as I heard/remembered. And I have scifi SB discs where the extra image clarity reveals some deficiencies of the special effects.