These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Sirocco, the hero dragon, is accessible on James' second mission. Provided you stay in good graces with Persephone, she makes the first half of the game ludicrously easy.
If you can get Frozen Ground and Bore then that's a complete Game Breaker. The developers had the sense not to let anyone get both those spells in the campaign, but you can make custom wizards for multiplayer . . .
The raise dead spell costs as much as the cheapest spell in the game, casts quickly and without freezing the player in place and has a fairly long range, making it ridiculously easy to keep your army in the fight. Compounding this, the only real way to win, most of the time, is by capturing souls from your opponent — and a quick trigger finger with Raise Dead makes that almost impossible by bringing them back to life near-instantly.
Mind you, Raise Dead * does* come with what is probably the worst or second-to-worst melee fighter in the game...
And a necromancer's shield spell has a short duration and long cooldown, making necromancers the easiest wizard to kill or disable.
As sympathetic as she gets, Shakti never ends well. (With the possible exception that if you serve James, you get no indication that Persephone or either of her wizards died, so she could still be alive then. More likely, though, the news just hasn't come through yet.) The most sadistic punch is on Stratos' 6th mission.
Poor, misguided Sorcha. No matter who you serve, she's going down, and unless you're serving Stratos, you're the one who snuffs her. If you serve Pyro, you even kill her just after she finally Defects from Decadence.
Charlotte fares no better. She dies in the final mission of James' campaign, is killed by the player in Stratos, Pyro or Charnel's campaigns and presumably dies offscreen in Persephone's campaign.
Considering Eldred was using her altar at the time, she most likely survived in Persephone's campaign since James would be using both his champions to defend Agothera.