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Although I'm a fan of the Dark Elves, in this case, I'll add my two cents in favor of the Light Elves. When playing as them, the hero always gets Luck and Defense skills upgraded (at least, I always upgrade them). Moreover, if you don't aim for super-powerful abilities, you can upgrade Magic Resistance and Magic Reflection (I'm not sure if that's the exact name) + unicorns create something like a magical reflection aura around themselves (again, I'm not entirely sure, I guess I'm getting old). This significantly increases the chance that an attacking spell won't work at all (personally, I always get annoyed by this "spell deflected" message – it makes me want to smash the computer!). If the spell does work, its effect will be significantly weakened. Therefore, it won't be easy to wipe out Sylvans with magic. As for the strength of the units, they are comparable between the Light and Dark Elves.
Level 1: Assassins are stronger than Dryads (in my opinion).
Level 2: It's more complicated here, but if the Furies don't reach and attack on the first try, the Dancers will tear them apart (again, in my opinion).
Level 3: There's nothing to discuss here – Hunters are the best, Minotaurs are weak.
Level 4: The situation is debatable again, but personally, I would rather fight against Riders than against Druids (with any army).
Level 5: Again, Hydras are stronger than Unicorns, but Unicorns have magic resistance. Okay, let's give the victory to Hydras.
Level 6: Ents are definitely stronger than Sovereigns.
Level 7: Well, the Black Dragons are probably better due to their immunity to magic, but in general, they are comparable.
So, that's the situation – the Light Elves aren't as weak as they seem.