It’s handicapping because it alters your teammates based on your own performance within your rank. Yes, the games are 50-50, but it slows progression toward your accurate rank and reduces match quality.
By matching overperformers with underperformers to make an equal game, you increase the chances overperformers will lose despite being in a rank where they should win. You decrease the chances underperformers will lose despite being in a rank where they should lose. Given enough time, everyone gets where they belong with PBSR, but this drags it out and creates an addictive system where every bit of SR is hard earned.
I go into more depth here: