I'm really disappointed by the wording of Radagast's text box :
You may use resources from Radagast's pool to pay for Creature allies of any sphere.
This ability doesn't allow you to pay 0-cost cards as you don't have a resource match. Notably, if you use your Staff to reduce the cost of a Creature by 2, you will not be able to play it if you don't have another hero with the correct resource icon.
This is half of the available creatures cards in the pool, and the ones you want to play the most : Messenger Raven, Vassal of the Windlord, Winged Guardian, Eagle Emissary, The Riddermark's Finest, Wild Stallion, Bill the Pony. (the ratio gets even worse with AleP cards).
Most of the time Radagast is played in an Eagle deck alongside heroes, so you won't notice it, but it harms seriously other build like a Grey Wanderer deck.
A good fix would be to recognize that, as every other Istari, Radagast should gain a printed icon :
When you play a Creature card, Radagast gains the printed , , and icons.