The spell compendium version guarantees a critical, not the successfull attack, you still need to hit.
.Your first hit with a bow (not including crossbows) in the next round is automatically a critical hit. If you don't hit in the round following the casting of this spell, the effect is wasted
See the wording. 'first hit' and 'if you don't hit..spell wasted' SO you do a hit and it is automatically a critical, no need to roll a successful roll.
also.. balance and 3rd don't mix very well, so try not to reason or use logic on the 'why' behind balance decisions