I'm now midway through a game with Mongolia. Didn't get a great roll on city-State neighbours, with 3 of 5 within reach being Maritime. If you force annex on those ones, the massive

dump can sink your capital pretty hard. I annexed one to see, and it grew from 12-17

on the yields alone, and put me in a minor poverty hole that took me ~25 turns to get back out of. I stand by my assertion that Mongolia's kit is just too powerful right now. The annexations require very little effort, can be done in fairly quick succession (as long as your happiness can handle it), and the yields bath is huge. Religious, Cultured, and Militaristic city-states are a massive boon. I'm not sure why the 250% on heavy tribute is necessary; Temujin already gets a whole city with all its infrastructure intact. Just the normal amount, or +50% would be more than enough.
Mongolia went from having no yields on a 50 turn cooldown annex to having 2.5x yields on annex AND they can now fire it off basically whenever their empire can manage the happiness hit. It's a massive steroid.
I'll be damned if it's not a tremendously fun kit tho. Massive border blobs, an almost guaranteed religion, and 2 obscene power spikes at Mathematics & Physics. You're kickin ass and taking names, running laps around everyone else, especially with a desert nearby. Lots of fun.