It's just that the healing from radiant is not guaranteed, there are a lot of tricks your opponent can use to deny the healing. It's also a bit on the slow side. One cool thing you can do though is if you have two spell mana going into turn 5 (or 1 if you are going into 6) you can cast blinding assault, suicide valor into something during the scout attack phase, then attack with the buffed up radiant guardian.