Low CPU overhead. VNC streams screen grabs with minimal (if any) compression, which results in low CPU overhead, high bandwidth consumption and low frame rate. This is great for the use-case of low-level VM debugging that it's used for in context of virtualization management systems, not so great for desktop remoting.

While RDP may run okay on 56k with low color mode for some use cases (e.g. simple Windows admin), it requires significantly more bandwidth and compute overhead (either CPU or GPU) for other more advanced use-cases (e.g. video editing, CAD etc.)