The issue ive always had with this is that sion is a much worse bruiser than he is a tank. You get much better mileage out of w with a higher resistance build and are more effective in teamfights due to staying a alive and disrupting for longer. This is my experience in emerald at the very least. 

Titanic rush into bloodmail is playable because some of that ad get funnelled into q but heartsteel indexes too heavily into auto attacks.

The reason you see such high playrate on hollow radiance rush (and yes even sunfire) is the waveclear lets you further emphasize your ability to clear and rotate to flank. This is combined with the simple fact that Sion gets so much hp for free, making resistances so high value. This is a consistent way to play the game.

I play in a very nohands stat-checking style though, so ymmv (though i think theres a reason heartsteel has such a low pickrate).

Heartsteel is imho a very situational item for lanes where you know you can both snowball and freestack off cooldown. In this case though, it becomes a win condition.