They will face pretty much the same problems you would face with having loads of poor revolting land except they will not have your powerful human mind to deal with it. Not to mention they lose half of their tax income to you (that comes straight out of their coffers and into your teching money)
They will likely have terrible tech and be piss poor until the land cores. If they have land which spawns lots of rebels or land that they do not have a drect connection to they will almost certainly lose that land to rebels without help.
But if you nurture your massive vassal (who will never betray you unless you break the alliance, which can also happen if one of you switches religion) they can be a very powerful helping hand. I had a 52 province Brittany help me out mssively and they kept growing through colonisation.
They will likely have terrible tech and be piss poor until the land cores. If they have land which spawns lots of rebels or land that they do not have a drect connection to they will almost certainly lose that land to rebels without help.
But if you nurture your massive vassal (who will never betray you unless you break the alliance, which can also happen if one of you switches religion) they can be a very powerful helping hand. I had a 52 province Brittany help me out mssively and they kept growing through colonisation.