Monitors aren't meant to be offensive ships. That's why their attack value is very low. Though it maybe could use a slight improvement. The attack value on ships essentially is their ability to attack in close quarters (boarding, close-range gunnery, or ramming). A typical Monitor had a handful of men as crew (boarding is out), 2 360 degree turrets, and had no ability to ram a ship (these were small ships, without even a real deck, think of them as like a tin-can on water). The Confederate Ironclads though were equipped with iron "beaks", which gave them large ramming power. I probably should slightly increase their attack to illustrate this better come to think of it.
I would like to point out though, that Monitors sometimes WERE used as offensive ships for bombarding coastal cities/forts (Charleston being the best example), but they were very poor at doing so. However, they were VERY good at taking punishment (some ships took over 40 direct hits without sinking).
Ironclads were not per se an improvement in gunnery, they were an improvement in ship protection. A Ironclad should be able to knock out any wooden ship with relative ease, while being hard to knock out itself by a wooden ship.
For traditional ships, the attacker has a HUGE advantage, if the ships are comparable (say a Union Sloop vs. a Confederate Sloop for example). The attacker will almost always win.
If an Ironclad attacks a traditional ship, they still will almost always win (and with extra HP, have an even better chance). If defending, the extra HP combined with very high defense means they are hard to kill by any ship. Personally I see no reason not to at least build the Confederate versions. The Union Monitors though could use something to spice them up.
Kind of off-topic, but I liked how they increased the power of Ironclads in C3C, relative to Frigates. It was a good change. Though I disagree with their bombard being equal to a destroyer. That's a whole other topic though. The reason I brought this up though, is the Frigates in C3C are 1600's-1700's style, while Frigates in 1860 are more durable, faster, steam-powered, and equipped with larger more accurate guns. Ships of the Line, while not really used during this time period by any nation anymore, still would be capable of crushing a Frigate if given the chance (speed though is a HUGE factor in traditional ship combat). The PC game Imperialism I think very accurately portrayed the difference between Frigates, Ships of the Line, and Ironclads.