The problems arise not from strings, they are well understood computationally, and most languages provide sound functions for working with them. The problems arise when strings are held responsible for the side effects of other functions which manipulate the data within strings.

In particular, problems arise when some function renders a sequence of characters contained in a string [fundamental string operations such as concatenation tend not to be problematic]. These problems are due to the transition from the mathematical certainty of strings to the heuristics of text rendering. The compromises required to map semantics of human writing systems onto strings via Unicode contributes to this problem...glyphs are not necessarily ordered sequentially or without resolution under a context.