The rewards are based on the number of Treasure Maps you collect. However, if you wish to try your luck, you can also use your Treasure Maps to open Buried Treasure for a chance at immediate rewards. Note: that using your Treasure Maps in this way will remove them from your inventory and they will not count towards your event totals. If you decide to use all your Treasure Maps to open Buried Treasure and have 0 at the end of the event then you won't get any additional rewards.
Nope, opening a Buried Treasure will always yield something. What that thing is is up to chance, however.
I want the old hunt system back!! With immediate rewards from boxes -_- and unlimited opening of boxes
You can open the Buried Treasure as often as you want. However, doing so will not count them towards the total rewards at the end. The Nautical Crates have the limit on how many you can open using the regular key each day.