Welcome to THR Kellogg!
My FFL buddy has used Juggernaut parts, maybe a complete build kit, ill have to ask.
If your going with the Juggernaut lower, id probably go with the juggernaut upper. Either complete or a stripped upper and add parts. From looking at the sight, its got some mill lines that wont line up with your normal/other upper receivers. The slab sided BCA side charger might look ok, but i think most others would look "off"
Personally my builds usually start cheap, and end cheap, so i cant say for most of the higher end parts except when helping folks with their builds. IVE never seen a functional issue with using parts from different companies, but some of the more unique lower/uppers can look weird when used with parts that werent ment to go to them.