Period + np.timedelta64 doesn't raise TypeError(NotImplemented) (like offsets does), and results in erratic internal representation.
import pandas as pd
import numpy as np
p = pd.Period('2011-01-01', freq='D')
p = p + np.timedelta64(2, 'h')
# This raises no error
# TypeError: don't know how to convert scalar number to int
PeriodIndex + np.timedelta64 also doesn't raise TypeError(NotImplemented), and are added ignoring PeriodIndex.freq.