I have two seasons on my SC Gen2 with Enve 7.8 rims. The only precautions are:
- Use the "shaved" brake pads on the front brake (8mm) versus the full width (10mm) for the rear. The extra clearance is good for adjustability.
- Don't expect exceptional braking performance with the 7.8 textured surface and the SC brakes when wet. The SC brakes are "average" when dry and go to hell with carbon rims when wet. Give yourself time and distance. It is a known issue specifically with SC brakes and textured rims at Enve.
Other than that, the brakes sound like a jet engine when slowing down, it's a pretty cool sound.
As for the flexing, I have the Powertap version of Enve 7.8's so cannot speak to other hubs. The rims aren't the problems of flex, nor lacing or tight spokes. It is the location of the brakes relative to the frame which puts some movement into the system. You're going to get some rub when powering up hills, usually standing and climbing with rocking the bike. Expect to tinker with the rear brake width adjustment slightly (the 2mm adjustment screws on the brake arms) to get them close enough for comfort to the rims without too close for some rubbing when climbing.
EDIT to add: You'll get both sets of brake pads with the wheels, the standard and thinner versions, and really nice valve extenders with a rubber sleeve on the extenders.