A mesocyclone generates many effects as a result of the mixing of warm updrafts, and cold downdrafts. The main effect is hail, which is almost always present if a mesocyclone has formed. The hail generates powerful lightning bolts. Other effects of a mesocyclone include powerful downdrafts, which can take down an aircraft, strong wind shear, and torrential downpour.
The most severe side-effect of the mesocyclone occurs when a downdraft and the flanking line of the mesocyclone collide. The first effects will be a "mammatus", or a bubbly cloud formation. If the downdraft is strong enough, the flanking line will lower and produce a "wall cloud". The final result is the formation of funnel clouds and finally tornadoes.
On the whole, mesocyclones are a dangerous, but common phenomenon.