I do this kind of stuff all the time, and, frankly, find this "hand-wave" of yours disengenuous. Do you even know what a PLL is? Have you ever used one, say, in a communication task where the data rate varies slightly? What about where you switch from one data channel to another, at a high rate and have only a finite length of sync field before you start to lose data?

what does two different clock speeds of a uC have to do with "a communication task where the data rate varies slightly"

how can "why not use a derivative with a PLL?" be considered a "hand-wave", I think anyone but Richard Erlacher would take it as a suggestion.

erik