Just how bad are bottle dynamos?
Bottle dynamos aren't actually as bad as often claimed. They are usually cheap, lightweight, have less resistance than people think and zero resistance when not in use. There are some bad parts, though:


Most of available dynamos are complete rubbish and won't last in use. High quality dynamos have been superseded by hub dynamos and battery lights.
Resistance is still higher than with decent hub dynamo
They are noisy and the pitch depends on speed, making any slowing down obvious
The cable connections are often very unreliable (see "complete rubbish")
The clamps damage finish on the fork. Some systems use fork and frame for ground and depend on puncturing the finish, preventing putting any protection between clamp and fork. This also doesn't work with carbon forks.
The dynamo is in perfect position to fall into spokes and can cause serious accidents if not properly installed. It is also easily knocked into spokes, so you have to check every time if you leave your bike in public place.