BPSK and QPSK's BER( In the case of coherent carrier)
The actual phase of the carrier during modulation depends on the start and end of changes in phase shift rather than absolute.
The method of demodulation must be determined before coherent phase error effects on BER may or may not be affected.
e.g. it depends on the mixer used for detected phase shift. If the modulation exceeds the phase detector’s 90 deg or 180 deg detection then the result is a reduction of Vpp phase to voltage conversion, i.e. signal level thus SNR ratio,  otherwise if it does not exceed by closing the loop with PLL to null the carrier phase error to the middle of the detector range then no degradation of signal would occur. There are many types of phase detectors from analog multipliers to digital XOR and D FF phase/frequency types which inject more jitter but wider +/-180 phase detection and other types.  The filter Q and group delay distortion also can introduce ISI in the detector so Raised Cosine filters with zero ISI would have to be assumed.