Well, Mega Gengar is noticeably more powerful than Scarf Chandelure, and doesn't have to worry about staying in while locked into one move. (Though, Chandelure has stronger secondary STAB moves.)
252 SpA Mega Gengar Shadow Ball vs. 0 HP / 0 SpD Gengar: 408-480 (156.3 - 183.9%)
252 SpA Chandelure Shadow Ball vs. 0 HP / 0 SpD Gengar: 360-426 (137.9 - 163.2%)
Still, Mega Gengar can switch in frequently and trap more Pokemon without worrying about a SR weakness. It also has better typing and a better movepool (Focus Blast, Thunderbolt, Perish Song, etc.).
Not to mention it can successfully trap more than half of the metagame. Also, it's a bit ridiculous to say that Mega Gengar lacks power. Of course it can't KO everything easily without super effective moves;
it has to pick off threats.
Disclaimer: I personally haven't battled against Mega Gengar, but it seems pretty darn broken on paper. Shadow Tag alone is downright uncompetitive anyway.