Playing in plat 3 today (solo, not grouped) I was getting around +34 LP for 1st place finishes, and -59 for the one 8th place, and -24 for the one 6th place. At that rate in order to advance from plat 3 to plat 2 I need to 1st place 3 games in a row. If I finish less than 5th in any game I will then have to 1st place two more games in order to be able to make up that lost ground. Considering games can range from 25-45 minutes, and it's an 8 player game, requiring this amount of first place finishes to make up for any slightly rough game you have to just feel excessively punishing.
It just feels so obvious to have the 1st place gains match the 8th place losses, 2nd matches 7th, etc. that I feel like it must have been a very intentional decision to divert from that system. Makes me legitimately wonder what the reason is for the disparity. I'm open to a real answer because I'm baffled.
Ultimately my problem with the system can be summed up in a moment I had earlier today. I had gotten 1st, 1st, 2nd in 3 matches and was inches from promotion. In the next game I had a rough start and right when I was trying to pivot I got matched against the first place guy twice in a row and the the 4th place guy who was on a streak. As I'm getting pummeled I suddenly realize I'm about to get knocked out, glance at the scoreboard and my reaction to seeing I was gonna be 8th wasn't "Ahhh dammit. back to the grind" it was a horrified "Oh god! NOT 8th! Christ no! Not 8th! auuuuggghhhhh" because I knew how much that would set me back and how hard it would be to make up that progress (even 1st place will take 2 games of 40+ minutes each to make up the loss).
Riot should try to avoid putting their players in a position to feel this way. It's the kind of situation that makes a player turn the game off and go play Underlords or Hearthstone or something.