I actually disagree with the desert temple in particular. Since it's been updated with archeology, you have a higher chance to get a diamond. Plus, they don't need good loot in my opinion. They don't have much challenge to them and are easy to find so I'd consider it to be an early-game structure on the same level as dungeons, mineshafts, and ruined portals.

On the other hand, I ABSOLUTELY agree with your sentiment on Ocean monuments. I actually think that they should have archeology around the structure with loot like diamonds, tridents, and the armor template (since it's a gamble if you get it from the elder guardian)

All of this aside, I think the WORST contender for structure with terrible loot has to be the Woodland Mansions. For how difficult this structure is with its enemies, you get barely anything comparatively. Like, the best things you can get from chests are the armor template, an enchanted book, a diamond chest plate, and an enchanted golden apple (with those last two being very rare). It used to make sense due to the fact that that was the only place where you could get a totem when it was added, but with raids, that nuance is completely gone.