More specifically: Don't juse a high-level language to implement the delays with just a busy-loop with a magic loop count.

C is ok to use when the delay is long enough that it is meaningful to poll a hw timer or use an interrupt handler. And better C compilers should have correctly handled nop constructs to implement minimum delays (no guaranteed upper bound) of a number of instruction cycle lengths, for example when toggling a processor pin.

But a HLL should never be used to generate loops of fixed length without "binding" the timing to some hardware of known speed.