Plenty of low gears is one thing, but not being able to make advantage of even slight down hills would be irritating and I think it would lead to the bike feeling excessively slow.
Sorry I didn't mean I could hold 20 as an average!

However, given a decent anything from flat to slightly down hill (1 to 2% ish) gradually building up the speed to around 20 would not be difficult. The bike would require more power to accelerate at a give rate, but doubt it would differ much in terms of power required to sustain a given speed when compared to a similar, but shorter flat bar bike.