Typically, they lack zoom capability, variable focus, and any ability to customize the image.
Whether any of that is important depends on what you are trying to do.
If you just need to embed video of a conference or lecture to provide information to viewers, webcams might provide all the power you need.
For serious professional quality video, though, you should consider other options.
Some are more flexible than others, some are more user-friendly than others, some provide a higher quality image than others.