Ice pellets penetrate the de ice fluid and no holdover time is stated for them, therefore takoff should be prohibited.
Certain airlines are claiming that it should be Captain's descretion to take off anyways and are effectively ignoring the rule. Perhaps the FAA will actually do something about it before these guys have another accident.
Takeoff into ice pellets also exposes you to a high risk of encountering freezing rain at a slightly higher altitude. Not a good thing.