Simmons is an enigma, he has most of the tools to be a great player but a massive deficiency in the way.
re: Horford vs Harris
A. Look at their career stats, Al is superior.
B. Look at more than just scoring right now, Al generates offense for others, VORP etc
C. Defense, a big that can guard multiple people, guard outside is incredibly important to defend 3's
D. Likewise a big that can stretch defenses is vital and more valuable in today's NBA than a SF
Quick ranking search and this is first one I found, Tobias #40, Al #34 - if its as definitive as you say, find at least TWO that have Tobias higher or we can average them.
Ranking NBA's Top 100 Players of 2018-19 Season so Far