keep in mind they are all AR's but the specs are not always exactly the same. For example I'm building a Daniel Defense AR and i was just doing some test fitting on a cheaper Dalphon Industries lower. The two will not fit together but i put this same Daniel Defense upper on a Bushmaster and it works fine. I'm sure it's not common but it's just a example of the different specs. Just something to consider.
Me personally I'm getting a Daniel Defense upper so i'm going with a Daniel Defense lower as well to avoid issues like that.