One of the things I learnt this weekend is that with luggage on the front, I can't grind at super-slow cadence. Steering doesn't work. I need to be able to turn the pedals at a reasonable cadence or I end up weaving badly.
On an unladen bike I'm satisfied to sit down and grind away at a very low cadence but I found I just couldn't do this with luggage on the front. Not a case of lack of leg strength, just steering etc. If I were fitter, I could have gone faster, but . . .
This is similar to the recumbent problem: As your body mass is fixed in the seat, balance has to be achieved by manipulating the steering. If the cadence drops too low the amplitude of the wobble exceeds what you can correct with the steering and you lose control. Obviously the specifics of when this happens will vary greatly with bike geometry, but at some point you need a lower gear.
On the other hand, touring 'bents allow you to carry front luggage without loading the steering. Done right this allows more optimal load distribution (you can put the heavy stuff right down between the wheels), and makes for very consistent handling.
