Its a fleet booster. It synergies. Thats the Advent thing. You use it help break a tough defense.
I'm not even going to consider the cultural flip (and the associated military bonuses) since DE's are so easy to counter with culture centers...
It gives you a 25% damage boost for 2 and half minutes...that is not very long at all, and good luck trying to use multiple deliverance engines to prolong the duration of the buff during a battle...
Does it help? Sure...anything helps...but...compared to a kostura that can disable all structures (like repair bays, research labs, SUPERWEAPONS) for 3 minutes, I'd say that the DE could do a lot better in helping out assaults on fortress worlds...
If it lasted longer, or guaranteed a culture change so you'd get your nice shield mitigation, then maybe it'd be comparable...maybe...
But you don't get the culture flip for sure, and it doesn't last very long...By the time the cannon shell even hits the target, the enemy may have forced you to defend elsewhere, or an opportunity may have presented itself and the offense is now on a different planet...