I've been playing TW games on VH/H since... STW1?
And I am simply not able to finish a FotS game on VH for any faction.
The one and only reason is the ridiculous balance between naval and coastal guns. Historically, the coastal guns of this period were devastating, extremely effective area of denial weapon. Similar to the surface skimming SSM of our time.
Navies needed an overwhelming advantage to even attempt to take a fortified coastal city or port. Here it seems a single gunboat can not only out-range a coastal battery, but bombard the city with impunity.
Now, I know that we can just use our own navy to destroy these pesky 1 gunboat fleets of bombardment death. But at this point, 80% of the game seems to revolve around this. And unless you have one major surface combatant per port, you cannot effectively prevent it, since the AI sends an endless stream of these gunboats.
Solution:
I remember up until RTW, the campaign AI had a extremely annoying tendency of sending out "armies" of 1 or 2 armies, and this was before it was possible to "raid". So effectively, all they did was to throw themselves into a battle with no chance of winning. That was annoying, wasted the players time, but the game was still playable.
By ETW, the finally figured out how to get the AI to group their armies into real armies.
I don't know what was changed in the AI, but that change needs to be done for the navies as well. In regular STW, we also had this problem but it was a non-issue since fleets couldn't do more than raid trade lanes, and their effectiveness at that was minimal.