I'm sorry if I'm being silly but the wording has me slightly confused. I just finished an adventure and was awarded an UURL with a U as a club reward. I'm guaranteed an item "at least as rare as the rarest item I would otherwise be guaranteed" What does that mean exactly?... If I open a brown box I'm guaranteed an uncommon item but might get something rarer?
Say you open an epic chest: basic loot would be 3 items + 1 guaranteed epic (or better) item Club reward would be also epic (or better) If any of loot happens to be a Legendary item (say, you got a lucky drop) that doesn't mean the club reward also needs to be Legendary. Similarly for Magnificent Chests (the golden ones) the club reward is at least rare (regardless whatever actually dropped). You can still get a LLLL(R) drop from such a chest. Hope that clarifies it a bit. And yes, the wording is slightly misleading, but mainly because how convoluted the system itself is. I don't think you can explain it much better and more effectively than how it's done, in only a few words.
One more thing to confuse matters: Some adventures guarantee a certain item the first time you complete them, and the club reward will be at least as rare as that item. So you'll always see at least an Epic in the club reward the first time through each treasure hunt, and a Legendary the first time you beat Melvelous.
As long as we're confusing... Magnificent Chests are gold, but not all gold chests are Magnificent chests. Magnificent chests refer only to the gold chests in chest shops. It's important to know that across the game, what is guaranteed in a certain chest is NOT consistent by chest color! For example, even just within the MP reward ladder, the gold chests have different minimum guarantees than each other.
Since we're being confusing here, remember that some people use "gold chests" to mean "chests purchased with gold" which are actually brown colored. (And then there's the people who use "quest" to mean "adventure" and "turn" to mean "round".)