Here's why balance in 1v1 does not mean balance in FFA, nor does it mean it would be fun for FFA players.
Imagine there are 2 characters, Microchu, and Gigadorf. Microchu only has one attack, which hits the nearest enemy for 1 damage, and moves him to where that enemy is, and does virtually no knockback, unless the enemy is at 999%, then it gets a KO. Gigadorf only has one attack, super gerudo warlock punch, which always hits all enemies in the game for 999% damage and instant KO, but is very slow. Microchu wins 1v1, but Gigadorf owns in FFA. If you buff Gigadorf/nerf Microchu to fix the 1v1 matchup, FFA becomes even more lame. Yes, this FFA is lame, not just unbalanced, even for the casual player. So, in order for characters with moves like Gerudo warlock punch or Link's Up B to be balanced in FFA, they must be to week in 1v1, and for them to be balanced in 1v1, they must be too strong in FFA. Thus comes a problem, since with this diversity it's very difficult to balance. I admit this is highly exaggerated, but many smaller versions of this are going on. Thus, 1v1 balance != FFA balance.
I also support good in both singleplayer and multiplayer modes, although multiplayer is far more important, as after long enough, it's basically all most people will play, anyway. Singleplayer just will eventually get old, but I still find melee's multiplayer to be the most fun of any game still, even with how much I've played it.