>Your AC will suck massively
Forsaker gets +10 to DEX, +10 deflection AC bonus and +5 armor bonus by lv 20 before accounting for armor. That's +20 AC bonus unless I am mistaken. By lv 20 a forsaker could be rocking a 42-ish AC without much difficulty.
>you have no additional damage except for your strenght and power attack
They do get a Magebane enchantment on their weapons, and ignore hardness/DR. In addition, they get +10 STR(+5 damage), a +5 untyped weapon bonus(+5 damage) and some bonus combat feats so it's not like they are very much behind a Fighter or something.
>and you have no further immunities to stuff like demoralize and poisons.
Forsakers are immune to poisons and diseases since lv 6, and immune to fear since lv 3. See under "Extraordinary Immunities". Here is what they get by lv 20:
Immunity to blindness, deafness, fear, disease, poison, ability damage, ability drain, energy drain, paralysis, polymorphing effects (including petrification and turning into dust if slain by destruction), death effects, fatigue, exhaustion, sickened condition, nausea condition, dazing, stunning, all the immunities and abilities associated with mind blank(as in, he gains them, not becomes immune to them), and critical hits.
This is a pretty terribly over the curve class and it's author should think really hard about their behavior.
EDIT: amusingly enough, it can't fly or move fast. So as long as you destroy his bow and stay at least 60 ft away, it can't do anything.