in that one, at least, by the time you got far enough that you had multiple kingships and so many vassals, you had also worked on your genetics enough to have massive diplomacy scores, and a good chancellor on top of that, which kept it from being much of an issue, except when you get unlucky with some rival who also has other factors to make him disloyal. And even then, quickly taking him out and then giving back (most of) his titles generally does the trick.
But if the II is less generous in diplomacy ratings, I can imagine things get less simple... nice job convincing a heiress to marry anyone, though! I got to the point where I refused to marry off any of "my" daughters or sisters on account of them being far too useful as ever-loyal vassals whose lands will always revert to the crown upon their death - since I could count on them refusing any and all marriage offers.
On a somewhat related note, I saw a game by the same producers named Victoria (and Victoria II) on Steam not long ago, some kind of Europa Universalis variant set in the Victorian time. Does anybody have experience with that, is it as cool as it seems like it might be?