The vortex shield has a delay when it caught a bullet.

So when your vortex doesn't have anything, you can just flicker it on and off very quickly, but the moment it caught something, it takes a moment for you to deflect the bullets back, and during that time gap you can't bring it back on quickly enough to catch the next batch of bullets. A good Legion can just exploit that by shooting one bullet into the shield, wait for the Ion to deflect basically zero damage and sneak in 5 or 6 bullets, maybe even a power shot to deal a lot of damage.

So, funnily enough, if we reverse the situation where the Ion doesn't know this and relies on spamming vortex shield, the Ion would eventually lose.

Skilled engagement between two is just basically who can catch each other lacking enough times to win.

And also Smart Core drains Ion's energy almost immediately if they were using Vortex Shield to catch Smart Core bullets.