Dunno 'bout that... the Union had a fair number of duds!
Context, as usual, is important... not all ironclads were designed for the same mission. For instance, Carondelet and her sisters were primarily designed with a view to bows-on fighting, and were visualized as floating gun platforms for fighting enemy shore batteries; they had some weaknesses against other vessels that become important when discussing Plum Point Bend and the duel with Arkansas. Arkansas, on the other hand, was conceived primarily to combat other ships.
By contrast, Union monitors were in their element against enemy ships-- with their low profile and ultra-heavy armament they were intended to survive until they could deliver a limited number of telling blows to a protected enemy. But they were lousy at shore bombardment, where the targets are less heavily protected and also won't simply sink or explode with one or two shots in vital locations-- that's when you need a higher rate of fire from smaller weapons.*
Considering the technological and industrial base the Confederacy had to work with, casemate ironclads like Tennessee were a realistic and achievable response... though a strong argument can be made that the Confederacy should have deployed its resources more into mines and torpedo boats, and saved the iron to try to buttress the railroad network.
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* ETA I like to bring up in this context the experiences of the monitor Montauk at Fort McAllister. She could withstand the fire of the fort for a considerable length of time (though not indefinitely), but there was no single target in the fort that she could take out in one or two shots. But she did just fine destroying the Rattlesnake (ex-Nashville) in exactly the same circumstances.