I find conquering (most of) the world and maintaining the blob that ensues is fairly easy, it just gets really boring after a little while since nothing can stand up to you in a fair fight. Other than the odd large rebellion, which are easy to head off if you manage your lands right, there stops being any meaningful threat.
As Byzantireland, I found the bigger empires more than big enough to go toe to toe with me. Both the Golden Horde and the Seljuks were evenly matched, and the Fatimids + allies probably could have matched me too. I was marginally weaker militarily than the HRE, I think.
I eventually usurped most of the Golden Horde, which put me massively over the vassal limits, even with Imperial Admin, so I had two options: make a Ua Briain Golden Horde that may or may not collapse under my character's brother, which would be difficult due to their mix of religions and the AI generally struggling with religious insurgencies, or set them all free and retain the title of Khan for myself, which would allow the smaller nations to coagulate.
Eventually, I just retained Khan, then gave the Golden Horde title to my character's brother, resulting in the Golden Horde owning an independent Venice and nothing else, with former Horde members squabbling with each other over land rights (and eventually being bitten into by Byzantireland, Ruthenia, the Seljuks and (in the EUIV save) Oirat.