Why these exceptions to the rule?

There are many champions with windups or channel abilities (velkoz, kat, mf, fiddles poppy). Drop a stun when they start the channel or windup and boom, canceled. Perfect, im fine with that.

I would like to be explained how is the reasoning behind 3 different cases of AOE windup. Gnar, Darius and Sion.

Gnar (W on mega) has the shortest windup and range between the 3 champs im discussing, he gets stunned, he doesnt get canceled. Darius has a decent enough range, his windup is slightly slower than gnar's, but he can move, he gets stunned, spinning axe goes trough. Sion has the biggest range of all 3, but the channel is almost double of the time, not to mention he cant move nor use any other spells. He gets stunned, his windup gets canceled, his mana is not refunded and his ability goes on cd for 2-3 secs.

Some might argue that Darius is a juggernaut and he gets special treatment. Ok? Well, Gnar isnt. Some might argue that Sion cant be interrupted by cc on his ult. Well, thats an ult on ~100 secs cd.

So why Darius and Gnar are not treated like the rest of the champions on that regard? Its unfair.