Flows are the daddy of all rims IMO, especially tubeless. I have never had any issues with them no matter how hard i ragged them.
I was in the bike shop today to put an order for exactly what you were looking for and it turns out there was no way for him to order online the flow / evo hub combo in 650b from hope. There were availability for both crest and arch rims but not flow.
He tried to get a hold of hope to clarify but as he has a very good wheelbuilder working for him i just decided to get a set build up for the same price that the would cost from hope.
Bikeneil, i am sure they are ace rims but for a 650b wheelset they come in about 200g heavier than if built with flows. I know it doesnt sound like much but i would rather lose that weight and keep with something i know works very well.