I think Koko brought up a good point about defining what we consider "broken" to mean, first and foremost.
I would propose that focusing solely on Chandelure's power is a waste of all our times: there are stronger Pokemon in UU (Darmantian), there are better offensive pivots, (CB Flygon / CB Crobat), and there are Pokemon more able to sweep through teams (Kingdra, Mew). These are all considered to be some of the best Pokemon in UU, but there hasn't been nearly the support for banning them that we have for Chandelure. We have to think about why this is, and whether it actually does merit a ban for Chandelure over those Pokemon.
I personally feel that the reason for Chandelure's relative dominance is its incredibly relevant set of resistances. If Chandelure were Fire / Dark, for example, it would probably be languishing in mid to low UU despite maintaining essentially the same offensive presence. However, as it is, Chandelure is one of two viable offensive ghosts in the entire tier (Froslass and Sableye are support) and given the pervasiveness of choiced Fighting and Fire type attackers, its very presence on a team is incredibly threatening. Taken in combination with its immediate and overwhelming offensive presence (something Cofagrigus lacks), it's easy to see why so many people run into problems with the thing.
However, I am still unconvinced that this is a metagame balance problem; I personally feel it's more a metagame stagnation problem. In my view, UU has achieved a state of balance where a number of very powerful threats all keep each other in check and a number of niches become available to be filled by good-but-not-great Pokemon. This is why much of RU and even some of NU is so viable in the tier. Unfortunately, one of the issues with a balanced metagame is that metagame development tends to be very slow. People are hesitant to adapt new strategies because the old standards are all so comfortable, and we eventually see Pokemon which are perfectly adapted to their particular niche as being "broken" - an excellent example would be Venusaur in DPP UU.
To me, for a Pokemon to be broken, the metagame and teambuilding has to revolve around it to a notable extent and potentially to the exclusion of the rest of the tier (see: Staraptor, Sand, Chansey). The metagame does not revolve around Chandelure; Chandelure has simply found a spot right in the middle to occupy. A good way to tell the difference - when you build a team, do you worry about Chandelure sweeping you more than, say, Heracross or Flygon or Zapdos sweeping you? I can't imagine many of us would say "Yes".
Everything was beautiful. Nothing hurt.
I'm back, baby. Look for me in 5th gen UU!