My guess: you roll the weapon's S vs the enemy's I as their T. So Tau and Necrons and Carnifexes count as T2, being I2, while a Daemon Prince counts as T8, being I8. And so on and so forth. I assume still going to be S6 or S7 (but like it says, still calculating ID vs enemy Toughness).
So my spiders will hurt marines on a 4+...
That's a bit of a nut punch for them
Still buzzing about the rest of the aspects though
I'm thinking a cad with a farseer on a bike, five jetbikes with shuricannons as a bodyguard, some guardians, an aspect shrine formation and a dire avenger shrine formation should be a fun start for a list
In my head, I've gone back to the third ed craftworld codex - 3 squads of DA, a squad of scorpions, a squad of banshees and a squad of hawks.
With still room for tanks and a phoenix lord/avatar/more aspect warriors.
3 falcons with aspect warriors like fire dragons with no scatter deep strike seems a bit powerful - 3 pulse lasers, 3 shuriken cannons, 3 more heavy weapons and 18 melta shots or 21 with fast shot exarchs?