I disagree with the statement about tricycles.
A recumbent tricycle in particular is an excellent Audax vehicle.
Only slightly slower than a 'bent, and faster than a wedge, in my experience.
Comfortable, I can ride for 14 hours with minimal discomfort.
Efficient, aerodynamically.
Safe. Noticed by motorists, who are very considerate.
Stable. Hill climbing as slow as you have to, in a straight line. A very relaxed ride, because balancing is taken out of the equation.

Audax speed.
The overall minimum speed is 14.3 kph.
Some events may set a minimum of 15 kph.
There is a general maximum of 30 kph.