You mentioned all the positive, but not the negatives. One, it is a fire type, which means it’s weak to rocks, which is an issue. Second, all ghost types, not just Blacephalon, have to compete with Dragapult. Even though Pult lacks that much ghost power, it more than makes up for it with having great speed and access to U-Turn.
Aside from Dragapult, the metagame is very hostile to it. Rapid Urshifu, or Moistshifu as it is referred to, is one of the most common wallbreakers which with its priority aqua jet just deletes Blacephalon. Weavile, Tapu Koko, Tornadus and Zeraora all outspeed it and can revenge kill it. 107 speed just isn't that impressive anymore. There's also the fact that it takes 79% percent minimum from a resisted priority in banded grassy glide from Rillaboom.
This doesn't just apply to Blacephalon but all Pokemon in general. Raw firepower alone does not dictate what tier you'll end up. If that's the case then Slaking and Regigigas would be the top tier ou mons.
In short, Dragapult, extreme frailty, stealth rock, the metagame as a whole
