SMAC: Should I use Psi attack?

Psi attack/defense is orthogonal to conventional weapons. Its result depends on Morale levels of attacking/defending units.

If an attacker/defender is a Mind Worm, they have their own class, plus both faction's Planet (Green) scores/attitudes largely affect the outcome of the fight.

Answer: see what's your faction's Morale and/or Green score.

You may also trick the system by changing your Government/Economy type before the attack. Say, if you possess some Mind Worms and you are planning to give them a victorious ride, switch to Green several turns before the planned invasion. Or to Fundamentalist + Power if you are planning Psi attacks with conventional units.

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Personally, I love Green because if you are lucky enough, you can capture native life forms, making a considerable amount of your units Mind Worms (Independent) which means it requires no support from a home base, still performing as police in "at least one unit defending each Base" paradigm.

If you have dominant weapons, mixing in hovertanks and even air-dropped infantry will let you continue leveraging those dominant weapons.

Psi-combat is mostly useful when facing technologically superior enemies that your weapons cannot defeat. Switching from overwhelming firepower to psi-attack will make you lose as many (if not more) units, since psi-combat is less lopsided in general.

I'd say consider changing your gravs to transports and using their mobility to deploy slower, ground-based units that ignore AAA. At least until your opponents stop putting all of their eggs in one basket.