Having events require planing and hard work is great, but when the final "challenge" is a low probablility event, what fun and challenge there was in the other quests of the event looses it's apeal. In fact I'll go as far as saying it killed the enjoyment of the whole thing. If I wanted that sort of "excitment" I would be playing the lottery, not a strategy game.
To me there will be no sence of achivement in completing the event, if I'm not lucky and my explorers bring back the needed resources, I'll use the gems to get the final reward, in either case it will be as exciting as buying a pair of socks.