Necromancy Spells Warforged are not immune to:
- Causes Fear
- Chill Touch
- Ray of Enfeeblement (a personal favorite of mine)
- Blindness/Deathness
- False Life (another favorite)
- Scare
- Spectral Hand (of course, all this really does is deliver touch attacks)
- Vampiric Touch
- Bestow Curse
- Fear
- Symbol of Pain (if triggered, the warforged are not immune to it's effects)
- Circle of Death
- Symbol of Fear
- Finger of Death
- Symobl of Weakness
- Clone (like false life, not really an attack spell)
- Symbol of Death
- Astral Projection
- Soul Bind
- Wail of the Banshee
There are 41 Necromancy Spells
8 of those either target, control, create, or otherwise deal with undead and wouldn't target most PCs anyways, leaving 33 spells that target PCs, some of them beneficial. Warforged are affected by 20 of thouse, or about 60%. Also, warforged gain the benifits of all the helpful necromacy spells. So if there's a necromancer in the party, Warforged get to benefit from any helpful spells the necromancer may cast.
Of course, this is just one school. The perennial favorite, evocation, is still 100% effective. And then there's my personal favorite, enchantment.
A 25% chance of failure for Sneek attacks and crits isn't immunity. The main argument is that they are immune to to many things right out of the gate, but the main attack of assassins isn't one of them. However, there is that feat you mentioned.The feat, Improved Fortification, grants immunity to sneak attacks and critacal hits at the expence of all healings spells from the healing subschool.
Oh, and for whatever it's worth: Warforged are immmune to most of the effects of Eybite. If they have more than 10 HD, they are immune to it completely.