In the past, I too have been in the position of having qualified tax professionals fail to complete returns on time, and of having difficulty contacting the accountant during the busy tax season. All manner of excuses were offered at the time, from work pressures to bereavement (which of course may have been true).
I would suggest, if you are penalised for the late return, that you negotiate with the accountant to reducing his or fee accordingly.
Of course if the accountant cannot complete the returns (if you have failed to supply all the information required, in good time, for example), the accountant may not be to blame.