[ynfnehf]

How is that any different than reversing the English digraphs? "th", "ch", "wh", etc. Reversing English doesn't usually produce something meaningful either.

[nerdponx]

Some languages consider (or used to consider) "ch" as a single "letter".

[lifthrasiir]

You are entirely correct that that K&R exercise is flawed in the first place. I'm more concerned about the language-specific rules for (somehow) reversing a text therefore.