It's certainly not unusual to get rained out here in Michigan. But this year has been dry and I can't think of a single time a measuring day was rained out for me.
Like Bob, I doubt there's much loss of accuracy when measuring in the rain as long as you also calibrate in the rain. But I have no interest in testing this theory, and instead will just continue to tell RDs that it's not accurate, even though I suspect that's not true.