On their products page, it states that these billet receivers are machined from 6061T6 aluminum, on their standard receivers, 7075 is available at an additional price. It is my understanding that 6061 is not as strong as 7075, not as much importance on the lower (not much stress), but very important in the upper (very stressed, especially in the event of some type of failure, as rare as it may be).
Also in case you were not aware, like me up until about a month ago, billet receivers are not as strong as forged.
I am not sure I would go this route for an upper, probably OK for the lower, on their webpage they have a link to the Brownell's website and there are some reviews on there seems a little mixed.