A rider on a regular bike has poor aerodynamics in every direction as their cross section is not that much different with an open frame and spoke wheels. But a

A rider on a regular bike has poor aerodynamics in every direction as their cross section is not that much different with an open frame and spoke wheels. But a sleek enclosure would be like a wing and have very different forces depending on the angle of attack against the wind. A gusty cross wind could much more easily push the vehicle across lanes or even knock them over...