I'm not sure if you're being deliberately self-contradictory, or if you don't understand the meaning of the word. Efficacious: successful in producing a desired or intended result; effective. If not all are saved (they're not), then it is not efficacious for all, or else it was not intended for all. If it was intended for all, then the effect must be there, or it cannot be efficacious, because it did not bring about the intended effect.
If some people die of dehydration, then the water was not efficacious for all. It would be silly to say that the water saved everyone from dehydration, even though some did, in fact, die of dehydration. The water is only efficacious if it has the effect. If it is not used, then it has no effect, and it must necessarily not be efficacious.
If not everyone is saved (they're not), then the atonement is limited. The effect is the same no matter which side of the fence you're on. The effect is that not everyone is atoned for. The only question is why. In the end, most are not saved, for whichever reason. Either it was not meant for everyone, or else it was meant for everyone and failed to be efficacious for everyone. Either way, the effect is limited.