I would say AF is superior to UA in nearly all aspects in Y and probably W, although I gave up UA a long time ago for their mediocre service (my IAD-based parents say they are just as lousy today).
The AF Premium Economy has so-so reviews because the seat is in a fixed shell and doesn't recline, rather it slides down. So not quite good for sleeping. If you go into the purchase with that in mind, *perhaps* it might be not as bad as people make it out to be.
Flying Blue uses a dynamic pricing award chart so really, the pricing of an award ticket is roughly proportional to the cash price of a ticket. Wouldn't say it's a complete devaluation although for the higher-demanded days/seasons, it can definitely be one.