I’m definitely finding that fast vs. slow water makes a big difference. For the former, larger and brighter flies can work well. For the latter, small and more subdued is best.
Example: I tied earlier in the year some thin and small Baetis emergers in a size 18 (edit: recipe here). They work phenomenally well when the current is slow to moderate. They get lost in the wash in white water.
When fish are in tailouts or shallow water, I will continue to fish this BWO fly.