According to the actual british battle records (in Japan), those guns never actually were able to hit anything, and the only reason why they don't killed anyone was because they were used very sporadically (especially after some of them burst in relatively short engagement).
You're misreading. They did not actually burst.
While the vent pieces of some of them failed, that was expected to happen (indeed they carried spares) and a large number of shots were fired. To quote someone from another site:
Between 5 July 1861 and 20 Feb 1863, the gunnery training ship HMS Cambridge fired 316 shot and 81 shell with the 110pdr. Over these 397 shots, they encountered 8 jams, or an average of 49.6 shots per jam, none of which rendered the gun inoperable. You may protest that a training establishment would naturally be more adept with the gun than the average sailor, but see below.
It should also be noted that the results of the bombardment at Kagoshima bear out this approximate 50 round per failure figure. The Euryalus fired 67 rounds and encountered one vent piece broken off; the Racehorse fired 50 rounds and encountered 3 jams, one which delayed firing for 25 minutes and another for 10 minutes; the Coquette fired 37 rounds and the Argus 22. None of the guns were rendered completely inoperable, only temporarily so.
Which does not take away the fact, that she was VERY long, and have a VERY deep draft. Nobody ever suggested to use semi-armored frigates against coastlines, as far as I knew. She was initially designed as sort-of "armored fast vanguard" for liner's column.
If she sails slowly up to the southern or western face of Fort Monroe, drops anchor, and opens fire - what exactly can the Union do to stop her? She's impervious to every gun mounted there.
Er, there was rudder here.
The rudder's underwater at normal fighting draft, that's sort of how a rudder works. In any case, the rudder is at most as exposed as on every other ship in the world - how often did rudder damage disable wooden ships in battles? (The answer is - not very often.)
That's actually a minor peeve of mine. For some reason, when the Warrior gets into an alternate-history battle,
every shot homes in on the rudder - even when facing an enemy which, based on observed historical gunnery, would be lucky to hit the frigate...