Forsaken are best thought of as niche commanders. At low EE costs, they can deal more damage and capture chosen for longer than your demon commanders. But at high EE cost's, I think the more flexible demon commanders end up being better for damage. The more damage types a demon does, the more powerful it becomes overall. On the other hand, the more damage types a forsaken goes for, the less powerful they become. Additially, sometimes it's advantagious to only send out a commander for 4-5 turns, which isn't an option for forsaken. Forsaken capture duration is between 5-8 turns (4 might be possible, but I've not seen it get that low). They also can't ambush, so it'll typically be between 6-9 rounds before they become available to capture again, assuming you capture ASAP, and we aren't factoring in detonate. On the other hand, Forsaken can potentially capture more during a battle than a demon could, considering they have no capture limit. That might only be useful for getting drain % though, as capturing 3 times per combat should be all you need.
Forsaken can also be assigned to "punisher" positions, which allows them to perform a punisher action on a valid chosen, and apply a unique effect for doing so. These effects vary between different forsaken, based on their vulnerabilities as a chosen. You can find a full list in the "forsaken refrence.txt" doc. But, in general, they can be grouped into four categories of bonuses; Trainer, Empowerment, Breaker, and misc. Trainer effects confer various bonuses or buffs to the affected chosen when they fall (damage upgrades, stamina/willpower modifiers, EE cost reduction). Empowerment effects buff the forsaken performing the punisher (damage and stamina/willpower modifiers). Breaker effects harm chosen resolve during the final battle (initial resolve reduction, resolve damage increase). Misc. effects don't fall into any of the other categories, so things like bonus EE or easier punishers.
So High EE cost "Training" forsaken can be used to produce powerful forsaken. Empowerment forsaken can make themselves even stronger by punishing chosen. Breaker forsaken can shatter the chosen's will to fight - making the final battle easier. Keep in mind that all effects are permanent (as far as I can tell). The majority of effects are scaled based on disgrace, so higher cost forsaken will have a greater % effect than lower cost forsaken. Only one forsaken can be assigned to a punishes at a time, and demoting a forsaken will reduce their willpower by 50%. I'll also say that the wording on some of the effects could be clearer. So just ask on the form is something seems a bit confusing.
