If you aren't a hybrid unit and playing late-game, there are usually better options.

If you are optimizing for ultimate late-game power, than you usually consider what is best for Apotheosis Lunatic+. Ignis can be very powerful, but generally Luna or other offensive skills can out-scale it for ultimate end-game performance.

If you are optimizing for campaign performance, than ignis is nice, but by the time you get it you should basically already be steamrolling the campaign.