If the video has 10 frames per second, which is on the low end, that leaves 0.15k per frame. Also the frame rate is usually reduced, depending on the scene. And finally, the video is compressed with a lossy compression scheme. However this is still a relatively small tradeoff for the ability to view a video within seconds. The quality will improve as the bandwidth improves and as modems get faster.
Quality is the first main disadvantage and availability is the second.
This prevents all UNIX users from using these formats. Currently the number of Windows and Mac users far outweigh the number of UNIX users, so this can be tolerated. MPEG files are quite large compared to the streamed formats, but the quality is much better. This is a decision each developer must face. I would recommend starting with a streamed format, and if you notice a lot of UNIX users visiting your website, consider adding MPEG videos later.