If one will notice, the ones that use the "tier" system are always the ones that own the $2-5000 custom AR styles from various builders that THEY consider top tier.
My S&W M&P MOE feeds anything I put in the mags, including some pretty bad Russian steel case stuff that choked my friends 'top tier' brand.
My BCG is 'mil spec', my barrel is chrome lined (1/9) and it just plain works after several thousnad rounds. Not top tier?
To me, top tier is (like 1911's) is a way to get you to pay more for the same parts that most everybody is using, BUT I (my experience only) have found brand name does not translate into any better reliability and that is what I expect out of my firearms, AR (M&P), pistol (I have couple of Glocks), revolvers (Smiths and Rugers).
To me, they are all top tier.