We have a gold crystal event happening. I can't find any information about them so I want to clarify some stuff.

Gold Crystals are basically like an item farm event. But you don't automatically get the unit at the end, you have to buy them from the Marketplace. True?

How long does the current unit stay in the Marketplace? Do they only rotate when new gold crystal events happen?

How good are gold crystal units compared to DC, event units and premium units?