Concerning Naval invasions:

It is a major problem because now there are three theatres of war instead of a single unified camapaign map, that are connected only through naval fleets. So if the AI is incapable of transporting fleets it implies that for the GC the AI factions will be restricted to their respective theatres, incapable of reinforcing one theatre from the other.I doubt that you think it desirable that I can besiege and take over Paris,France while the AI retains 20 stacks in new France.

Also since the AI is ignorant of the fact that it cannot transport units, it tends treat Islands as normal provinces building up large stacks that then do nothing for the remainder of the campaign except suck up army upkeep and will only come into play If and whwn the player decides to invade.

Read a more detailed expose in my thread:

https://forums.totalwar.org/vb/showthread.php?t=114186