It's not really difficult per se, but rather accentuated to a certain direction, which only certain type of approaches are made valid.
Search the Encyclopedia and look for any lord without their own fief. They will 9 times out of 10, will listen to your suggestion. If your relationship isn't too negative, they will come over easily. But other than this, convincing a lord with fiefs to come over, is pretty rare thing to happen.
I've seen it happen, done it a quite a few times -- lords with fiefs in peacetime, will come over leaving their fiefs behind. Lord with fiefs during wartime, will come over with their fiefs, instantly turning their fief into part of your kingdom. But like said, this is a rare thing to happen in general. In most cases, lords that have their own fief, will stay loyal. Reasonably speaking, it does make sense, but gameplay wise, it gets kinda frustrating at times that you have to pay hundreds of thousands of gold to bring over someone with no fiefs into your kingdom.