The Black Medalion doesn't fail. You have to understand that it takes effect even on the Pokemons you mechanically skip because you're not interested in them. Having 10 Snivy and only 1 Hidden Ability doesn't mean the BMS is broken. Let's take an example :
1.000 Pokemons Encountered
BMS 25% h.a. which means that you've encountered 250 hidden ability Pokemons
And in those 250 they were 100 Rattata, 90 Raticate, 30 Chansey, 19 Jynx and 1 Snivy.
You just encountered the 9 other Snivy among the 750 others non h.a. Pokemons.
Conclusion ? The Black Medalion is working, no doubt about it. The Hidden Ability hunt should remain a bit hard, and farming it at a rate of 25% is way enough for me.
Hope you understood my opinion :)