The lack of such markers prevents Unicode from encoding strings of mixed Japanese and Chinese text correctly. Or in the case of a piece of software that must accept both Chinese and Japanese names for different people, Unicode is insufficient for encoding the written forms of the names.

Just in case this has to be said: the reason this hasn't been a problem in the past is because you could solve this problem by picking a team and completely breaking support for the others.

With rapidly improving single-image i18n in OS and apps, "breaking support for the others" slowly became non-ideal or hardly viable solution, and the problem surfaced.