You can get 1 credit point per day ONLY so this will not replace farming, it is just a login bonus.
7 credits will give you a random reward from a *better* list of rewards.
14 credit and 21 credits again the same as 7 credits.
28 credits will let you CHOOSE the reward from the list.
The quest difficulty will be easy, it is designed for all levels above 10.
Bare in mind, we don't know how likely it is for these combat mounts, class changers and collectible costumes to appear on that list.
Let me remind you of your chances when you try prophetic card events. How often do they appear (when not at a checkpoint).
~ Glitchy, bearer of bad news