The main problem arises from the disparity between the effects on creatures and on players, I think. Most creatures will only gain a little HP from this, likely the same as or less than Plate Armor... and it's very uncommon to get more mileage than out of Heavy Armor. On the other hand, players can heal for 50HP, which is a half-Miracle, without "charging up" as GG points out.
The difference between this and other effects is that it is multiplicative. Plate Armor, Holy Light, Guardian Angel... all have fixed values, even Stone Skin has no multiplier among its maths. There is a huge difference in HP between players and most creatures, and that translates to a huge x huge = GARGANTUAN difference in the multiplied effects.
tl;dr - Holy Light will heal a Golden Dragon for 10 or a player for 10. This card will heal a Golden Dragon for 5 or a player for 50.