Yes but when you are tied up you can see it coming -
When Egwaine was tied to the chair and the white cloak was cutting Perrin , had she been aes Sedai , seems she would just channel and kill him. But he had no fear of that and obviously that had not happened before because he had killed many sisters and taken their rings
I hope you don't mind if I bring some book canon into this. Spoilered if not. But in the books, Aes Sedai can only do a weave the way that they learned it. It's said that someone can tell which Aes Sedai taught another because of specific hand movements used. In the show, it's referenced at least slightly. In the scene in... season 1 episode 2, I believe? The cold open with the Whitecloaks where Valda kills a Yellow Aes Sedai. He chopped her hands off because there are some weaves that she couldn't do without the use of her hands as a sort of focal point.
You see a roundabout reference again in season 2 (can't remember which episode) where Alanna is teaching a group of Novices that includes Egwene and Nynaeve. Egwene insists on doing the weave without using her hands because "I didn't come here for easy." She'd already been taught that using her hands for weaves was easier, but because of her time being captive by the Whitecloaks, she wanted to be able to do most of her weavinig without her hands. Just in case.
And also like Boreas said, they can only defend against what they can see coming. There's also the potential use of poisons such as the case with Moiraine being poisoned by a trolloc blade. Granted that was a trolloc and not a Whitecloak, but the same general rules apply.
Here's the thing. If it comes to a direct confrontation between two fully ready and able combatants, the AS win every time. But all a Whitecloak needs is a suspicion, and then they lie in wait. They take the AS and her warders unaware, knock her out and immediately kill the warder if they can. Then they make sure the AS is in too much pain to focus weaves continually until her death. In reality, this is another testament to Egwene's strength of will. She will not be broken by pain, and it is the thing that makes her the epitome of AS.