here's a couple of potential disadvantages:

blackout: Split-prism focusing screens tend to turn black in the center with slower lenses (usually f/5.6 or slower, depends on the screen). Unless you're using a slow lens, or setting the aperture manually, this probably won't be a major problem.

effects on metering: Focusing screens can affect the camera's metering system, especially in spot metering mode. I haven't found this to be a major problem when I've used focusing screens in the past, but it depends on the camera/screen combination. Check the katz-eye website for your specific camera, they list the effects of their screen on metering.

visual distraction: The focusing screen breaks up the image in a way that can make it harder to see the final image.

cost: The katzeye focusing screens are expensive.

other issues: There are some other possible issues with split prism focusing screens, like a darker viewfinder and alignment issues, but I don't think they're as likely to be a problem with a katzeye screen.

I've used a couple split prism focusing screens on different cameras and have liked them, but I haven't used a katzeye.