The first time I've seen gameplay of Jam 99 was through a video discussing the "forgotten" Jam titles. You're absolutely right about the game being one of the worst basketball games out there! I guess one of the only good aspects of the game was Kevin Harlan's appearance in a basketball game prior to the NBA 2K series which a lot of people don't know.
Another interesting thing a lot of people don't know; the game being absurdly fast is actually an emulation issue as many uploads on Youtube have gameplay taken from an emulator. I found a video taken from real N64 hardware which shows the game at its intended speed which makes it much easier to follow but no more fun to play!