No offense, but anyone telling you that Ninja is harder to play than DRG shouldn't be listened to.
They're on opposite ends of the spectrum in difficulty.

As for Ninja vs Monk, I'd say Ninja is harder to master. In terms of overall usability, they're really simple. A single dot tied to their slashing debuff, a speed buff that may as well be permanent and a dmg buff that IS permanent. A resource that's gathered simply by attacking normally, and never falls off and they're by far the melee effected the least by downtime.

Really the only difficulty, if you can call it that, is Trick Attack timing. The rest of Ninja as a whole basically is on auto pilot.