Riposte is the only spellshield that can negate:
15 abilities at once, assuming no one uses their ultimate.
It appears not to be a bug, but it's unreasonable that Fiora's spellshield is infinitely stronger than the others, with the option to stun multiple champions.
Edit: Well it appears people just think bringing up other, just as unfair abilities justifies this. While some others have no idea how targeting a champion works.
And really? In one comment someone said the duration was short enough to be fair but also that the root it gives her is long enough to matter. Those are contradicting points. You can downvote and bring up other broken champions if you want, but that doesn't change how the game works. No champion attempts their burst in more than a second. It's unreasonable to say that "if a player uses all their spells during a second then they don't know how to play" when that's the exact play style of all assassins.
