Xin Zhao is not as easy as you say. He is currently broken, but that is not due to his kit, in fact they just nerfed his gap-closer, so now it doesn't decrease CD. The reason he is broken is the same reason Yi, Jax, and Udyr, and Xin Zhao are practically the only junglers right now. Devourer's + Guinsoo's is stupidly strong on them. The truth is, Xin Zhao and Master Yi are all engage, if no teammates follow up when you go in, you will be dead, and I've had that happen a lot. Jax is really strong, but he relies on effective combination of his abilities to work. Udyr is easy.
But it is inaccurate to say that Xin Zhao is broken, or OP. Xin Zhao and Jax have balanced kits, but Devourer + Guinsoo's makes them insane.
The scenario you described also relies on 1) available, gankable squishies with little to no mobility, 2) low warding or map awareness, and/or 3) allies to help ensure you can get the kill. Remember, ganking should never be done alone, junglers get too little experience or gold to be sure they can win the fight. (Of course, some level 2-3 junglers are stupendous, like all the above mentioned, +Lee Sin +Sejuani and maybe a few others. Still, you can get destroyed if you try it alone, or your ally can't get to you.)
~CrimsonCobra3