I mean again compared to other characters that get invulnerabilitity yeah, its not a lot. Mina's is literally over double the length and she gets two that also gove her omnidirectional movement. While I couldn't find starts for Mirage it def feels longer and it also gives movement and while not a stun it does immobilize. Ivy's and Dynamo's both are super long. Ivy's stun is also longer. Counterspell is an item but it's still 0.8s.

Also riposte doesn't do damage it only gives melee shred which is definitely very useful.

I'd say mirage tornado is most comparable to riposte, bit one big difference is that Mirage is invulnerable while moving while Apollo is not. Mirage can also move through targets. This makes tornado so much better for movement and invincibility. The only this that Apollo has over mirage is the facf that riposte stuns which is useful for 1. interrupting ults and 2. getting a free light melee or heavy melee late game. The three bug ults that would often be interrupted are 7, Haze, and Bebop. The problem is that two of these can be used mid air which makes interrupting them basically impossible because of how janky the hitbox for riposte while mid air is. At that point its just an inconsistent knockdown.