It’s not so much character length from any specific encodings. It’s all the details that go into supporting it. Can’t assume text is read left to right. Can’t assume case insensitivity works the same way as your language. Can’t assume the shape of the glyph won’t be affected by the glyph next to it. Can’t assume the shape of a glyph won’t be affected by a glyph five down.

Pile up millions of these little assumptions you can no longer make in order to support every written language ever. It gets complicated.