Yes, but also no.
The reason why the AR15 platform is so popular is the fact that you’re able to customize the rifle however you please, and it’s quite easy to DIY.
While some people prefer buying a completed rifle, others love to modify the gun either for accuracy or for show.
The real trade-off in mix matching the parts (which to answer the ultimate question, yes you can mix-match), is truly time & money. If you choose a part that’s cheap because it was on sale, but choose a corresponding part that’s high-quality, you may not have the match you anticipated, therefore you’ve now wasted your time and possibly, your money.
You would also need to consider that different brands have different designs, some being more sleek and some being more traditional. What happens if the different parts have different logos engraved on them? What if “black” looks like two different shades when matching the parts from two different brands? It’s hard to make these decisions when shopping online which could also leave you with a look that you’re not particularly proud of.
It’s best to buy parts together, from a reliable supplier like AR15 Parts when customizing your AR build to ensure you’re receiving quality and consistency.
Does mix-matching affect the accuracy and precision of a custom AR?
Although combining upper and lower receivers of different brands will make no difference in accuracy, the quality of your upper receiver will. Accuracy is determined within the interface of your BCG, barrel, and up.
Quality must match quality when using different brands when building your AR.
As long as parts provided by your manufacturer are designated as Mil-Spec , you should have no issues interchanging upper and lower receivers.