Speaking from personal experience on the Ganondorf levels in Legend Mode, Ghirahim mostly seems to pull his weight, but dear god Zant is incompetent.

It kinda flip-flopped for me. At one moment, Zant is mowing down waves of baddies while Ghirahim was whining for help, then the next thing you know, Ghirahim is going toe-to-toe with an enemy warrior just fine, and Zant's getting swarmed.
I'll still love Zant, though.