Flaws here in the weakness.
I disagree with less usefulness. Only thing i think chandelure will have problem with usefulness is against current leader.
Other leaders is actually really good. Even angie because chandelure have flame burst, which destroys the field.
Actually chandelure have really decent to good. From the pre evolution, it gets: Confuse ray, will o wisp, flame burst, memento(useful based on situastion in the battle), shadow ball(which destroys your tm thing there).
For fully, pain split is a pretty smart move. Much bettter relearn instead of full evolve.
It gets overheat as tm from amber fyi which is very useful for the hecka 145 sp.atk.
Also hidden power is a thing, which is already useful for chandelure.
Breeding it's only heat wave so is useful.
Dusk stone can be find on 4th gym story time which is before even getting the pokemon itself.
For the advantage i agree expect being fast. Many other fire pokemon is faster than chandelure's speed is decent at best.
If i would rank it myself, it would be A- or A.