Electric boilers are extremely energy inefficient compared with heat pumps. If you built a Passive House, not going the extra mile for a heat pump seems silly to me. Not to mention that heat pumps can easily protect you against heatwaves (should any happen).
Exactly what coryrc said in his comment, for a PassivHaus you recover very slowly the cost of the heatpump and if it brakes once in 10-15 years while it takes to recover the investition... you will never recover it.
