Custom cassettes cannot be built without sacrificing some of the "hyper" in glide.

The positions of the shift gates are phased with the neighboring sprockets so the shifting chain meets the new sprocket with the spaces lined up with the teeth that have to engage them. When you mix/match you lose this phasing, and a shifting chain may meet the new sprocket with a roller lining up with a tooth rather than the space between them. This can cause some minor delay or skipping when shifted under a bit of load. This isn't a crisis,since we got along without gates for decades, but manufacturers don't encourage it since it's contrary to their marketing message.