I think the problem with most 20s and, 28s, come to think of it, is they don't weigh enough. I don't know too many folk who are skillful with a sub 6 lb gun, which, is what most 20s and 28s weigh. Back when McIntosh ordered his Spanish 28 of some sort, he was very specific that it weigh 6 lbs, which was the smartest thing I had seen when the topic of smallbores came up.

A 16 that weighs 6 lbs is pretty common, a 20 or 28 is not. 1 oz loads out of my 5 3/4 lb Remington 17 start to suck after, oh, about three of them.
Best,
Ted