Oh, I have THOUGHTS on this costume and the character's whole presentation. This costume was one of the things that, when I saw a glimpse of them before seeing the film, made me think, 'huh. can't wait to see how they make THAT work', and boy did they make it work.
1. He doesn't have a human face. If it isn't covered by the helmet, it is out of the frame, or blocked by something, or he's shown from behind. This sequence on the beach, shot in broad daylight, is where you can see him the most clearly, and still his face lacks an expression, and his eyes are black voids.
2. When he's on screen, you can always see and recognize him immediately: in a dark-ass frame, in total chaos of a battle scene, from the back, in the distance? Can't miss him if you try.
3. He's fucking terryfying. It's this uncanny feeling of something sharp, cold, unfeeling, not quite tangible, not quite human, invading and unstoppable. I'd say I distinctly perceived this character as an 'it', not a 'him'. A 'he' is a person, and this is something else. You can't reason with this Thing (well, maybe if you are Odysseus and you're doing what it wants anyway).
4. As in this film Agamemnon is not so much a character as an anthropomorphic personification of War, it is exactly right that its armour looks like a crazy amalgamation of historical and modern. War may change with time on the surface level - weapons and clothes and tactics and means change - but, quintessentially, it stays the same.
None of this is achieved through the costume alone, of course. It's a combination of the visual image, cinematography, the actor's work, other actors' work, music and sound design (especially that!) The overall effect is very effective in its clarity.
Hell, it even tracks that many people find him attractive (personally I'm not into this, but no kinkshaming in this house). Warmongering has always had good propaganda, and selling the construct of heroic masculinity to men through making it look sexy (and carefully constructing what 'sexy' even is around everything military) has been a working strategy for millenia.
And finally, this is a freakishly brave artistic choice on Ellen Mirojnick's part (and her team, of course). Whoever came up with it deserves a cookie, because they HAD to know how they will be drowned in shit from the very first promo shot. I applaud the nerve and the verve of everyone involved.