Summary — A well-made prestige drama that forgot to be about Green Lantern — the ring is set dressing and the Manhunters never feel threatening.
This is a well-made show about nothing. HBO spent prestige money on great actors and gorgeous production, the writing holds together scene to scene — and then forgot to make it about Green Lantern.
The ring gets used maybe twice across the whole season, never in a way that justifies the most powerful weapon in the universe being on screen. It's set dressing on a small-town drama that happens to star Hal Jordan. The Manhunters are hyped as an extinction-level threat and then just... stand around. We never see what they can do, never feel why they're scary, and the finale's solution to them is contrived enough to break the spell.
Strip the ring out and this is a perfectly good drama. But I didn't sign up for a perfectly good drama.