Slicing by byte-offset is pretty unhelpful, given how many Unicode characters occupy more than one byte. In an encoding like UTF-16, that's "all of them" but even in UTF-8 it's still "most of them".

Slicing by UTF-16 code-unit is still pretty unhelpful, since a lot of Unicode characters (such as emoji) do not fit in 16 bits, and are encoded as "surrogate pairs". If you happen to slice a surrogate pair in half, you've made a mess.

Slicing by code-points (the numbers allocated by the Unicode consortium) is better, but not great. A shape like the "é" in "café" could be written as U+0065 LATIN SMALL LETTER E followed by U+0301 COMBINING ACUTE ACCENT. Those are separate code-points, but if you slice between them you'll wind up with "cafe" and an isolated acute accent that will stick to whatever it's next to, like this:́

When combining characters stick to a base character, the result is called a "grapheme cluster". Slicing by grapheme clusters is the best option, but it's expensive since you need a bunch of data from the Unicode database to find the edges of each cluster - it depends on the properties assigned to each character.