alienist is not that strong, It is a class you take for flavor, because once your psudonatural things start showing up, you've gone beyond Marlboro country

.
The energy resists your summons have are not that good [acid electricity]and there is technicly NO way to communicate to a psudonatural creature to have it use its True Strike ability since there is no language that could be used to communicate to your summon.[celestial, infernal, and abyssal stoped working when they lost thier original template] 10+hd SR is cool though. plus they will already be pretty tough since you need augment summoning and 3rd circle summonings.
Summon Alien: "Whenever an alienist would use any summon monster spell to summon a celestial or fiendish creature, she instead summons a pseudonatural version of that creature. For example, by casting summon monster IV, she could summon a pseudonatural dire wolf. This adds the pseudonatural template (see page 160) to the summoned creature.
An alienist gives up the ability to summon nonpseudonatural creatures with a summon monster spell .For instance, the alienist described above couldn't summon a mephit or howler with summon monster IV.
But the benifits....
You will be quite the Uniter. Your spells will contact entities that demons and angels would unite to stop from entering the material world. You will feel that warm glow inside seeing Twin brothers, one a paladin, one a Blackguard, working together.... to gut you. Druids and rangers will keep tabs on you to make sure none of your
pets adds themselves to the local flora and/or fauna. You will flip the middle finger to everything the D&D cosmos stands for so make sure the DM is cool with it[but not too cool with it...], make sure the players are ok with this too, because if they stay in character as living beings in a sane world, they should kill you at first opportunity, cremate you and your spellbooks and scatter the ashes.
Most divine casters are not going to have a lot of love for someone who calls things from where the gods fear to tread. Arcane casters will be smart enough not to hang around you. The barbarian will likely wind up killing a lot of your summons because a few eyestalks looked at him
wrong.
But hey, at least you will eventuall stop aging and have no fear of dying of
natural causes. Unnatural causes are part and parcel of this line of work though. And since you ARE going to meet an unpleasant fate someday, why not share that with the rest of the world
. Chances are, that is why the DM allowed you to take this PRC.