How are unicorns stronger than magic elementals? Champions are definitely better in attack – they have higher speed and deal more bonus damage.
So, unicorns don't excel over anyone (except for the Scorpius) =(

Unicorns are stronger than magic elementals (if the latter are not in the hands of a specialist) because their damage against necromantic troops, as well as the aforementioned underground dragons, is noticeably higher. And they also have an ability that is effective against other units. Plus, they have more hit points.
In addition, Rampart has a whole range of extremely useful units. First of all, these are gnomes. Gnomes are valuable for the following reasons: they are one of the best units for producing demons; they can be skipped during construction to speed up the process, but if they are needed for something (like producing demons), they can be bred in sufficient quantities; they are excellent in terms of cost-effectiveness, both in conservatories and in utopias (where, by the way, magic elementals are not very effective without a specialist).
Considering the possibility of additional growth, their cost, and, of course, their ability, dendroids are a truly unique unit. They are excellent against fast-moving neutral units (this can be useful when capturing a utopia, breaking through the Main Guard, etc.), and they can also help stop an enemy's attacking force. How do you stop a horde of skeletons? Gorgons are not effective against them... But dendroids can provide significant help here...