Hitmonlee will be switching into hazards every time on top of taking LO recoil with each Knock Off, leaving it to be worn down very quickly. In addition, it can switch into nearly nothing, meaning a Pokemon will often have to be sacrificed just for Hitmonlee to come in and Knock Off Doublade each time, so it can eventually spin, probably costing itself, all for Yanmega to come in at full health. Obviously this won't be the case for every match, but Hitmonlee only being able to do so little with Knock Off is definitely not in its favour.
Even though hazards are easier to remove now, the fact that gen 5 RU had significantly less checks to Yanmega is extremely important because it means that Yanmega was forced out less; by not being forced out as often, the entire basis of the most relevant argument against Yanmega, its weakness to Stealth Rock, was rendered much less effective. In gen 5, hazard removal support was more difficult, but more importantly, it was less necessary, so Yanmega was easily a S rank threat without the support, and therefore constituted for S rank because it simply didn't need as much support as it does now. Even among the new ways to remove hazards, Defog basically guarantees a death on your team if the opponent has a Braviary, and as shown with the calcs above, Hitmonlee's spinning capabilities usually aren't enough to get past Doublade.
If you really hate Doublade run E-que on Hitmonlee, knock off + E-que kills Doublade:
252+ Atk Life Orb Hitmonlee Knock Off (97.5 BP) vs. 252 HP / 0 Def Eviolite Doublade: 135-161 (41.9 - 50%) -- 0.4% chance to 2HKO
252+ Atk Life Orb Hitmonlee Earthquake vs. 252 HP / 0 Def Doublade: 208-247 (64.5 - 76.7%) -- guaranteed 2HKO
5th gen RU had a lot of more counters and checks to Yanmega than XY RU:
5th gen counters that XY don't have:
- Aggron
- Aerodactyl
- Nidoqueen
- Clefable
- Klinklang
- Manectric (unless bug buzz)
- Bronzong
- Registeel
- Togetic
- Aromatisse
- Cobalion?