Completed a game as England recently. Forget the Longbowmen and its often ineffective range thanks to terrain features. The SotL are probably the most insane UU in the game.
Admittedly in the game I played, all the enemy capitals were coastal cities. However, I find that the unbridled power of the English navy is potentially game-breaking, considering I dominated both land AND sea using the ships alone. What happened was I finished military academy, spammed SotL which could by then be built in one or two turns (internal trade routes may have helped), conquered my whole continent, and then mass-upgraded to cruisers to take on the other continent, where civs like Babylon and Korea were about 10% ahead in literacy.
When the Battleship upgrade came, indirect fire plus the range promotion and logistics allowed me to wipe the entire land army of opposing civs (and the entire world was at war with me). This was in 3/9 where garrisoned units were immortal too. The power was such that I needed to build no new Battleships, only Destroyers, because the SotL turned Battleships have the power to wipe land armies like a normal Battleship can't.
I think that it might not have broken the game if the Ironclad line also needed iron (and the whole point of the ironclad warships was that they had iron armor anyway). Instead, the iron requirement came too late at Missile Cruiser, by which point you probably have already won the game with your Cruiser/Ironclad composition. Therefore you can have near unlimited navy in this game. The fact that naval units are stronger than land units due to mobility notwithstanding, they are not any harder to produce.
~Rule Britannia, Britannia rules the waves~
I did mention that I'm a British Historian, didn't I?

G