BUT, even the small UNSC ships often carry nuclear weapons. UNSC Frigates and Destroyers carry between 1-3 nukes, capable of destroying a huge amount of covvie ships at a time, what would those do to GTVA and Shivan ships which have minimal weapons and are clustered close together. Even the tiny prowlers carried (14?) nuclear mines. Ghost of Onyx even had a prototype of a new nuke which essentially blew the atmosphere off and destroyed half a planet(!!!) MAC rounds (piece 600 ton slug fired at 120,000KM/s + ships current speed) would probably smash most GTVA ships, and thats then the UNSC Defence platforms fire 3000 ton slugs at about half the speed of light, those destroyed Covvie ships AND any ship behind the target. Archer Missiles on most ships, Pillar of Autumn had 7800 missiles it could of fired at pretty much the same time. (although, the PoA was heavilly upgraded).
The effectiveness of nukes against covenant ships is negligible. One of the generals in the halo novels was said to have fired a nuke at a covenant cruiser with almost no affect against it's shields, at the sacrifice of an entire squadron of longsword fighters who were distracting it.
I'd like to know where you heard that a nuke could destroy multiple covenant ships, as if that were the case, the UNSC could win any space battle by simply using nuclear weapons properly. (I.E. mines, the method described above, etc...)
Also, as I recall, Harbinger and Helios torpedoes are the equivalent to a nuclear weapon in power, making the UNSCs nuclear weapons no more powerful than the GTVAs bombers. The harbinger according to the tech database has 5000 megatons of yield, substantially larger than the bomb dropped on hiroshima and probably larger than Shiva or FENRIS warheads.
EDIT: After doing some research, it seems that an average sized nuclear explosion would be enough to destroy a covenant cruiser if it was a direct hit, though seeing the massive size of the Covenant, it's unlikely that the UNSC has enough nukes for every Covenant ship to make them a viable weapon.
UNSC MAC weapons do seem like they would be rather effective against GTVA vessels, seeing as how they can be fired at extremely long ranges with very powerful yield. Though I'm sure the GTVA could invent a refurbished beam cannon design to hit at those ranges (or perhaps beam cannons can already reach that range, as evidenced by the Lucifer's ability to destroy entire cities on Vasuda Prime from orbit).