So, I gave a lot of thought about this which is one of the most frequent questions made. And rather than giving a simple yes or no answer, I reckon this requires some longer considerations.
Now, first of all Im still learning, and havent finished the game. But from what I have experienced so far, I feel I can add my 2 cents.
So, Hirge i obviously pretty good for the overall survival. If you have enough mana pots, you are unlikely to die with here - pretty handy indeed in special if you're playing IM.
However, this game aint just a matter of "how do I kill X Boss". On many occasions, the Boss - even strong ones - end up being the less of the problems.
The fact that you dont naturally heal, needing to sleep at predetermined places or using potions, means that progress in the game is a matter of endurance. What this means is, to keep advancing while minimizing resources spent in the least amount of time.
On several dungeons I went for instance, just getting to it proves an ordeal - which means a lot of potions needed, which means more gold spent for that purpose.
As an example, I just completed Hirge's quest of the Seal. And her part of it was super easy. But I decided to push foward.
I did reach the center of the Forbidden Pit, but that costed me a total of 83 mid mana potions, 17 mid hp and 3 elixirs of galade. But I decided not to try the Void Lord, since Hirge was dying to Dark Presences even with both Heal and Intervention 3.
And to be honest, I wasnt exactly safe either. Hirge only aggros 1 enemy at a time, because she doesnt have Cleave. So, sometimes when there were 2 or 3 shadows, they would go after me instead of being pinned at Hirge. That's one of her weaknesses.
At those moments, a quick Combustion+Disintegrate made short work of those - but again, that's heavy mana.
On other moments, she did die against the strongest shadows, leaving me to run like hell using Combustion and Disintegrate.
Anyway, despite those few moments, she did good. Im not so sure that Griss would survive on some occasions where Hirge did. However, with Griss it would've been a lot less potion demanding.
Therefore, I'd say that Hirge is pretty good on certain places, where she can greatly help to deal with foes that would be too strong or problematic otherwise, but Griss is overall far better at the long game of endurance. With Griss you'll spend far less in order to keep advancing, thus she'll in fact help you advance more and faster than Hirge, but Hirge allows you to overcome stances that would be too hard otherwise and potentially letal.
One of the main problems of Hirge is that she has a sort of conflict between her Intervention and Heal.
Without Heal, she wont heal YOU, and thus she becames an option a lot less safer, since her ability to save you is one of her main assets.
On the other hand, having Intervention with heal means you cant just unequip her to "regenerate" her like with Griss, because she heals herself up before losing enough health for that to happen, and Intervention simply prevents her from "naturally dying", consuming her mana instead.
And Hirge without Intervention is not good either, since she dies way faster and easier than Griss, without Intervention she cant fulfill her main purpose of tanking strong foes.
Thus, for Hirge to be truly formidable and a good option, she needs both Heal and Intervention, but that means she becomes a Black Hole of mana potions for the late game (early game not so much), and thus Griss becomes the default option for runnind around in your errands, while Hirge is still good for places too tough (althought, I still prefer to use hp pots and even scrolls of restoration for Griss).
For example, while I do pretty much everything with Griss, Im planning to use Hirge to deal with the Elemental Lord and that Demonic Prince of Sloth at the Forsaken Fortress. Those are two targets where Hirge can be truly helpful to deal with in a much easier way (or less insane at least).
And Adaon is really only situational, and for those veterans that really are up for an insane challenge.
