I don't quite agree that it fails to get through the most common walls, for example it wrecks ferrothorn, chansey/blissey, skarmory, umbreon, ttar, garchomp etc. The ones I can see it struggling with are bold alomomola, mantine, milotic, possibly impish mandibuzz and clefable. Those one are fairly easy to counter with serperior and other electric pokemons or they can also easily be walled by alolan-muk and ferrothorn (except clefable if it knows flamethrower).
Keep in mind those are mostly counters if they are trained in defense, if not Conk can easily break through those if it has a status condition with guts and facade.
The most mayor counters are togekiss and azumarill.
The fact that it has low speed isn't really that big of a deal since it kills for example greninja and weavile with mach punch, the weavile will most likely use a priority move in those cases which gives you a free opportunity to use drainpunch and heal up.