II like not having to use my own units for the event, although I hate the gold option to buy more. So far (box 22), I've only run out of a particular unit once, and I was able to finish using different missions.
My biggest issue with the event is the meager rewards, and the steep decrease in the odds. I like that we aren't getting full myth nukes as a reward, but only getting 4 or 6 harpies or mantis is pathetic for the time involved, and the inability to go back an reopen a particular box that you like.
Next time, keep the free units, but allow players to use their existing units (always a good way to use up units you don't want any more, especially with the decrease in BP from attacking allies), along with the gold purchase option.
Also, up the awards to make it more worth playing the event. And/or provide a choice of awards for various boxes, so the player can grab what they are interested in (myths, resources, favor, etc).
Finally, I do like that a larger range of players get awards (top 50), instead of just the top 3 players.