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