Yeah Mongolia is not a fun Civ to play, its UA funnels you into a tight scope of strategies, unless you just want to play a normal game and make use of the Keshiks but not your UA, which is possible.
That said, they are great to have as AI opponents. I like to play my games with a lot of interactions with city states so when I see Mongols around, I know there will be trouble.
EDIT: IMO one of the mistakes civ5 did was trying to make their leaders play to win like if they were players playing chess, instead of acting like civilization leaders
I hear this alot, but I don't recall previous Civ games; leaders having more character. In fact, in the Civ3 preview video, the point is made that when you meet the AI leaders, they start off in character but immediately jump out of it as game progresses.
The AI has been programmed to attempt to win forever, it's just that Civ5 took it to the next level. AI is a lot craftier in Civ5 and constantly plots behind your back.
Saying the AI is playing to win also implied the AI is all the same, but it isn't. Each AI has a preferred grand strategy. Such as the Huns and Mongols preferring early wars on one extreme. And Venice, Greece, Austria having a lock on city states. With the rest falling in-between. They all pursue their individual quirks and UA specific strategies in conjunction with trying to win.
The Agenda layer for AI leaders in Civ6 could satisfy you, but the core idea of having AI play to win is a nice one to have. You end up with more epic games than passive AIs who had all the tools to win but lets you win anyways.
If an AI that could win fails to win, there has to be a reason, such as being at war. Otherwise, it feels empty.