I am always blown away by how perfect the Beach Boys harmonies are. Not just the note-perfect harmonies, but the way they are blended. Part of the reason they sound amazing is that they sound so much like each other... but too much of the same sound doesn't work either.
Sometimes it is difficult to tell Dennis from Mike, Mike from Al, Al from Brian, Brian from Carl, Brian from Dennis, etc. This is mostly in the vocal stack. But with all the similarities, a wall of Brians does not sound the same as the original blend. None of the touring bands sound like the 5 original guys. What nuance is missing?
Of course, the guys also have their distinct voices as well that are easy to pick out but it has always intrigued me how nobody can precisely match their sound even when singing note-for-note and sounding a lot like them.
Even later in The Beach Boys' career, that harmony stack didn't sound the same. This is likely due to the changes in Dennis and Brian's voices, but while there are moments when that greatest can be heard (California Saga: California and the closing of Endless Harmony come to mind), it was never the same as it was in the 60s, going into the 70s.