T9 is harder then anything in FCoB.

T9 is 4 different fights in one. Almost nothing carries over from one phase to the next.
Phase1 - beak, chariot, dives, meteors
Phase2 - Golems and more meteors
Phase3- Towers plus ghost
Phase4 - Claws, Elementals, Chariot, Dive Bombs

T12
Phase 1 Phoenix plus Black/White Fire
Phase 2 Phoenix plus Brand/Red/Blue Fire
Phase 3 Bennu farm
Phase 4 Phoenix plus Brand,fountains,pinions

Phoneix moves never really change so its 2-4 people doing mechanics while 4-6 just fight. With current gear levels the learning curve is fairly low and is more of a mechanics check like T8.

T13
Phase 1 - Flatten,Megaflare,Earthshakers
Phase 2 - Flatten,Megaflare,Rage (Orbs)
Phase 3 - Adds plus easy divebombs
Phase 4 - Repeat phase 1 plus Twisters

Very low learning curve with mechanics but requires min/maxing healing,DPS and tank rotations. Its more of a "can you play well" check then anything else.