Drawbacks

City traffic and short distances

In city traffic and short distances, an upright bike wins on maneuvrability and oversight. Experienced velomobile riders don't have problems though riding in traffic.

Climbing

Due to the higher weight of a velomobile, ~ 50-80 pounds, climbing is somewhat slower. This is a factor on longer climbs. In a hilly, up and down type of ride, a velomobile has less of a problem, or even an advantage: the higher speed downhill can be used during the following uphill, leaving only a short stretch at lower speed. If climbing is a factor, you can consider electrical assist.

Price

A velomobile is considerably more expensive than a normal bike. But in many cases this is offset by mileage. You have more rideable days in a velomobile and you can cover greater distances.