For a lot of people, keeping vassals above 60+ relations is "really difficult" so they get "constant factions."
So their spy is working to force them out of factions, they're keeping some money saved in case they need mercenaries while they bribe their vassals to stay loyal, and they're marrying off their children to obtain alliances with strong neighbors and key vassals, right?
If they are unable to hold a kingdom together at all, then they need to ask for assistance as to what they're doing wrong. I've found my vassals can sometimes be challenging and I do have to actually work to keep the important ones appeased, but even when I get a revolt that knocks me from power it doesn't seem like feudalism is broken. It might seem unfair that your vassals might jump on you when you are at your weakest, and it can take a while to get to the point where you feel you have control over your kingdom again, but that's a good thing to have in a game.
And what's the point in playing a game about feudalism if you remove feudalism from the equation?