I'm divided in opinion in terms of faction reappearances.

On the positive side if you play the game because you enjoy the strategic battles, faction reappearances can be quite a breath of fresh air in the campaign games, due to the fact that many battles see you battling away against masses of spearman and other low level units. Faction reappearances give you the chance to go up against some of the games more advanced units, especially when they appear in high and late. The re-emerged faction is normally led by a 3 or 4 star King, which also gives a reasonable challenge.

The problem with this is that the armies can often be quite imbalanced and can often contain too many good units to make them practical. In a sense the army is too good for its own good. A re-emerged army can often contain up to 8 units of feudal knights and about 2 royal knights. Like the French often found in the Hundred Years War, armies like this can often be defeated by lesser men who act more professionally. Armies are not always like this, however, especially for factions where ranged troops are elite. English re-emergences are normally a pain because they can have a fair few longbow units later on.

The re-emerged factions would be better too if they also appeared only in their own homelands. I once had a Sicilian re-emergence in Wessex because they held the region for a few turns once. What would've been more exciting and perhaps realistic in the game for re-emergences would have been if the rebellion was comprised of the troops trained by the conquering faction in the conquered provinces and then a few simpler base units in the homelands. As it stands, the game seems to allow the re-emerging faction to train advanced units on little more than hope and prayer while established ones can take decades building up the required facilites. This way it would be like a true revolt of the people. In Medieval times, the nobles were normally happy to accept new masters as long as taxes remained the same and they were allowed to keep thier lands.