The problem with crystals is that the clock signal starts tiny and grows to normal size over a long time - several ms. During that startup time, the chip sees something out of the clock cell, but it's not a reliable clock, since the input is so small. So it gets clock pulses, even before the clock is full size. Adding a bit of noise may produce clock pulses that are too close together. i.e. too fast.
The chip only waits for about 128 us before releasing the micro. At the end of this time, the clock may still be too small to be reliable, so a fast clock cycle may occur and cause the micro to fail.
Another thing to mention here is that, again because crystals take time to start up, by the time crystals reach its full scale amplitude, a watch dog reset could have occurred. This is not a big deal at power up because we do not differentiate a POR or watchdog reset or Bus reset (under firmware control, you branch a Bus Reset interrupt to a system reset like POR). However, there could be a problem when the part comes out of suspend. During the clock restart at Resume, the host expects the part to be good, but the part may have gotten lost by then.
Resonators tend to start in about 50 us, so those are not a problem.
There is an additional circuit in the part that holds off the start-up counter until the clock amplitude reached a certain level. However, while the chip has been shown to work with some crystals over some conditions, we found that at least some customers will eventually have combinations of crystal / silicon / board layout / operating conditions that make it NOT work. So to avoid problems, a resonator is required.
With that explained, it's up to the customer to try a crystal, or implement an external RC circuit that holds the micro for a curtain time to give their crytal enough start up time.
Note that there is an internal 30pf cap at each xtalin/xtalout pin. In an AC analysis, this will give you an effective 15pf capacitive load. There will typically be 2-3pf of stray cap in the logic. Therefore, the total capacitive load that an oscillating element sees is about 18pf. The 18pf load cap is a common recommended load cap for many oscillators in the market. So, you should choose an oscillator that requires an external 18pf load.