Some skills do not have any equivalents (magic, weapon and armor skills). But:ID Item, Repair Item - B30 = E15 = M10 = G10, training the skill higher brings no advantage.
Disarm Trap, Perception - B40 = E20 = M14 = G10, training the skill higher brings no advantage.
All these skills follow the formula "base = 1x, expert = 2x, master = 3x, grand is always successful". The highest "difficulty level" is 30 for ID Item and Repair Item (and both values are always equal) and 40 for Disarm Trap and Perception.