[2020] I’ve noticed that during the Ghosts of Dathomir event, only certain characters are capable of scoring critical hits against the Spirit Altar, why is this the case? The two I’ve noticed are Thrawn (when he uses fracture) and T3-M4. What’s so special about those two?

Affirmative. The idea is you can't critically hit a slab of stone (no "weak spots"), but you can critically hit the spirits inhabiting it with special damage.