That is a slow projectile. Sidestepping all three is hilariously easy unless they're point-blank.
Lux's Light Binding
- 13-9s cooldown
- 2s root
- 70-250 + 70% AP damage
Morgana's Dark Binding
- 11s cooldown
- 2-3s root
- 80-300 +90% AP damage
Senna's Last Embrace
- 11s cooldown
- 1.45-2.25s root
- 70-250 +70% AD damage
It has the cooldown of Morgana's root, the damage of Lux's root, and a lower duration than either of them until max rank. It's the worst of both worlds, plus a delay on the root itself. It's objectively the least consistent of the three.
Her movement speed is the same as most champions, but her autoattack cast time is one of the longest in the game. She spends a long time standing still with each auto, so she can't be moving around and be a threat at the same time.
The reason why you can engage on her after her root ends is because if she runs away from you, she's dealing no damage to you. The root is barely more than a second at rank 1. Senna can only travel ~450 units in that time, and that's only if she doesn't follow up on her own root at all. Most dashes can cover that distance easily. So if you save your dash until the W is down, you can still close the gap. Or just don't get hit by it at all. It's a slow projectile.
Lux can follow up on her (longer) root with a slow to get even further away. Morgana can follow up on her (even longer) root with free damage while walking away unhindered with Black Shield. Senna has no other CC than Dark Passage, which has a delay, and her only mobility is a minor speed boost after a long delay that's on a ferociously long cooldown.
You only need one person to catch her out and kill her. Any assassin with half a brain that knows how to engage when important abilities are down can clean her up easy. A burst mage who waits and watches for her to root herself for half a second can land a full combo on her and take her out. She's not helpless, but she has reasonable counterplay.