No ability has really been banned before and I do not think inconsistent should be the first.
One weak side of this ability is that the users have weak base stats.
Another is that they share common weaknesses, and can be punished by pokemon which are very high on the tier usage charts.
If you always switch into an inconsistent counter when the inconsistent pokemon comes out, the +2 Evasion should be no worry. (it also has to decrease somewhere)
The most common weather seen in this generation is Sandstorm, and that is a huge counter to inconsistent pokemon. Substitutes usually take only 13% of the hp (Sub (+6 % leftovers) Protect (+6% leftovers) . But Sandstorm makes each sub 25 % , making the total amount of subs usable up to 3. This means 6 (+2) increases and 6(-1) decreases.
Enemies:
DW:
Breloom
Breloom is very effective counter to Inconsistent ability.
With his new ability, Technician, he can easily use Bullet seed to tear apart Bibarel and Octillery with their respective substitutes.
Also, Breloom can use Mach punch to do away with Glalie, although Bullet seed would work too, since Glalie is super weak in defenses.
Cloyster
Cloyster will come in right when the inconsistent pokemon switches in.
He can use shell smash, and Rock Blast will spell doom to all 3 users.
Roobushin
His combination of Drain Punch and Mach Punch is deadly. He can use Drain punch to break a Sub, then use Mach punch before you can hide behind another one. Seeing that Glalie and Bibarel are weak to this typing, and Octillery is not strong defensively, Roobushin is a great counter to this ability.
Zapdos
Rain is a common weather this generation, and it makes thunder is 100 % accurate.
Octillery and Bibarel are both weak to thunder, and glalie is weak in general and zapdos' high base special attack will tear through them.
Zapdos is also bulky enough to take +2 Waterfalls from Conservative Bibarel in the rain.
UU:
Sableye
This is mostly a counter for my team as a whole , which isnt posted here, so I'll just say it has priority taunt.
Morobareru
The ultimate weapon against this ability.
Seeing that many pokemon last generation, such as magnezone, have become OU due to their ability to counter certain pokemon,
Morobareru should do the same, rather than have the ability inconsistent banned because some players claim they cannot counter it.
CLEAR SMOG. CANNOT MISS. RESETS ALL STATS. DOES STAB DAMAGE.
I simply do not understand why people do not run clear smog on any 1 support pokemon to stop this ability, but insist on banning it instead. Morobareru has a 4x resistance to water types, meaning Octillery and Bibarel are 100 % countered. Not effected by toxic because it is part poison.
Glalie cannot get close to OHKO'ing this, even with +2 sp atk boost. Ice breath will do 70 % or so, and ice beam 80-90 %.
(depending on EV spread on both pokemon)
Regeneration makes morobareru a strong support pokemon, and it can carry double statuses (spore and stun(t) spore)
Finally Giga drain will heal itself while harming Octillery/Bibarel.
Chirachiino
Basically, its wide skill link movepool of Bullet Seed and Rock blast will cover all the inconsistents.
Sorry about the format, this was taken off my analysis of my own team. Tiers based off of Pokemon online server.
Other counters include perish song users of any sort.