On-board oscillators tend to be very inaccurate (as compared to crystal oscaillators, that is). In general, it is not much more than an RC-oscillator. The actual oscillation frequency will vary widly over termperature and applied voltage. If you want to have any assured accuracy at all, don't use it.

Resonators provide better accuracy, however they don't come close to that provided by (well layed-out) crystal oscillators.

Therefore, if you need high accuracy, don't even think of using anything else than a crystal oscillator for your clock.