This shit with the new damage classification is SO messy. Some skills do damage where they didn't before, some now do even if they shouldn't considering how they're classified etc...
EDIT: i fucking finally figured how it is supposed to work:
Spell piercing : if not, spell won't have ANY effects.
Spell piercing : if yes, spell will have effects but damage depend on the type.
This is not consistent with what they said (where they said that spell piercing don't actually block damage, bullshit it does), but it's the most consistent way to make it work ingame. The only problem is that some things change because of this. In general, if you want a spell to have pure damage and apply debuff, it will damage BkB users whereas before it didn't: example, Meat Hook now damage bkb/rage targets, howl of terror now apply debuffs even to bkb targets whereas before it didn't. If you want a spell to deal damage to a target (physical for example), it will have to pierce spell immunity or won't deal damage, example: crush now deal no damage anymore cause it don't stun so even if physical it deal 0 damage. Acid alche spray however now pierce bkb so it still deal damage as physical spell.
This mean that an eventual item that give only spell immunity (and not the 100% magic resist that bkb give), would be different from bkb only for those piercing spells with magic type damage, which are: Beastmaster ulti, Huskar ultimate, Mist Coil, Chain Frost, Mana Void, Lion Finger and Lina Laguna, Phoenix Supernova, Earth splitter, Epicenter, Assassinate, Requiem of souls, Viper strike, Puck ulti, Zeus ult, Black hole damage, Scythe etc...
Basically most damage ultis. Imho it wouldn't be worth it, it's even more confusing than before, unless they make Pure do what intended, damage BkB targets even with no piercing. So we get Pure/Physical to deal Damage to bkb regardless of BkB piercing, but not applying buffs/debuffs. Slar stun still work like it did etc...
Techies now also megasucks. 25% less damage against creeps mean he can't farm with mines anymore whereas before lvl 4 mine + 1 autos on every creep cleaned a wave. Fucking IF making a shitter hero even worse.