Even if you just look at the cleansing, then Cyndane is an isolated example. Demandred, Aran'gar and Graendal were overwhelmed by circles of lesser skill and similar force.

Battle and dueling are areas where finesse with the Power counts the least. It's mostly about beating up your opponent as quickly as possible.

Egwene's ability to split her flows 14 ways is possibly the most irrelevant thing you could mention regarding her ability to fight Nynaeve. Almost without exception, channelers place their whole potential into a single flow when they're fighting one another. Cyndane even sees how Alivia put her whole strength into a flow which she managed to slice. Do you really think Egwene would start with fancy weaving if Nynaeve sent this sort of assault after her. It was the same with Graendal's attack at the cleansing, and Flinn's.

Even more poignantly, Rand says that he can split his flows a dozen ways or more, but against Lanfear's attacks he needs to put all his power into a single weave strong enough to defend himself.

You can say what you like, but almost without exception, dueling in the series has demonstrated that channelers prefer to crush one another quickly with power. Skill is valuable in any field, but Nynaeve and Rand have managed to overwhelm huge skill by placing their opponents on their back feet and arm wrestling them. Essentially this is all dueling with the Power is in most cases. If a channeler has multiple flows toying around with complex weaves, she's about to lose or is dueling a much weaker opponent where she has that luxury. Winning duelists throw their might at their opponents, try to finish it quickly and don't mess around with complexities - the series has shown that all too well.