Seriously, they are too useful offensively and pretty strong defensively. Yeah, while many fairy pokemon do have laughable defense and or HP and thus end up easily dispatched by strong non resisted physical moves, the fairy power creep is just to start in Sun and Moon.

The proposed solution? Nerf the fairy defense by making their attacks interact normally with fighting type (instead of being super effective).

Sure, this will make fairy offensive ability nerfed a bit, but to be fair, fighting type is still at disadvantage against fairy offensively because the fighting moves are ineffective against fairy. And fairy is actually projected as a defensive type as well.

TL DR:
Fairy Offensive:
2x (Super Effective) vs Dragon, Dark
0.5x (Ineffective) vs Steel, Poison, Fire
Fairy Defensive:
2x (Weak against) vs Poison, Steel
0.5x (Resist) vs Dark, Bug, Fight
0x (Immune) vs Dragon