I long await the day that Halo abandons one hit kill sniping attacks. It wasn't good in Halo 2 and it still isn't good. Jackals I can learn to deal with, but the Warden and his near uncounterable OHK abilities was overkill in the most literal sense (we can't even shoot the tracking orb, really?!)
Legendary should increase the number of enemies, their aggressiveness, and the velocity and accuracy of their shots. But extra damage and extra health on top is something I'd rather they keep to the skulls.
It doesn't make the game harder, it makes the encounters less interesting because all that open space is wasted when enemies kill that quickly. I've never liked this about Halo and it's on full display here, even in spite of the leeway with the revive mechanic.
The Sanghelios levels in particular were beautiful open spaces, but running around them meant dying to just about anything in a blink of an eye. It got old crouching tiger hidden dragon behind chest high railings and peek shooting enemies with a Storm Rifle, only to die from something on the other side of the level that just got dropped in when you went to pick up a better gun. Even using vehicles felt pointless at times when enemies could destroy it so fast.