I don't know about half...the only spells that are unusable are (mass) sleep, mind control, charm, and pacify. Sleep and pacify are pretty useful, but charm can be worked around (by defending or going for another target), and mind control is too expensive (for me, anyway). In comparison, there are 21 force spells that the forcemaster can still make use of. Psychic spells are a relatively small part of the forcemaster's arsenal.
In the forcemaster's favor is the fact that it is rather difficult to give the undead unavoidable attacks, so defenses, block, and reverse attack are very effective.