Chest Trap Mechanics and Thief to Ninja Upgrade

*tl;dr* How do the inspect/disarm chest traps work in Wizardry V? Does upgrading my thief to a ninja make those functions harder?

I'm playing Wizardry V again after a long time away. I beat it on SNES as a kid with some help from hints on the puzzle, but I'm realizing my play style was really basic. I do remember one party attempt where I switched my thief to ninja and then was disappointed that he could seemingly no longer reliably inspect chests for traps or disarm them. Was that impression accurate? Is it necessary to be a thief to be good at this, or is there some stat combo that would work even on another class?

Thanks!