The 'shared' memory uses extremely slow system RAM (relative to dedicated video RAM). Compounding the problem with a video system built into the CPU occurs when data has to be moved to/from system by the video portion. It causes the CPU to have to stop working due to the data bus being the same for the video and CPU. Only one can access system memory at a time.
Shared memory, is a bad thing for a video system built into the CPU. The enormous delays in draws will cause things not to be drawn as the game is not going to wait forever for a frame to be rendered before moving to the next one. In a real-time environment, one cannot wait.