Most of us in this forum are riding folding bikes because we prefer them. It is true that as the wheels get smaller they tend to be twitchier, but that is something you just get used to. With twenty-inch wheels I find it hardly noticeable, while on 16 inch it more noticeable. I think wheel size is the determining factor, not wheelbase.

These bike store sales representatives just don't seem to know folding bikes.

If the price of the Bike Friday does not put you off, I say go for it. There are much cheaper tandem folding bikes on Amazon with similar geometry as the Bike Friday.