It's more carryable than SR at least.
Much easier to direct and carry 1 bad teammate than 2-4... even on the new map.
That said, 1 bad teammate can literally screw you over on SI (Shadow Isles) much easier than on SR or TT. If that teammate isn't willing to listen to advice you will easily lose all map control and any chances to win the game (because alters and no wards).
I had game once where our Garen decided it was a good idea to constantly roam the enemy jungle alone and try to cap alters while the entire enemy team was alive and close by. Needless to say he died doing this repeatedly and we would have lost the game if not for our Teemo that shroomed our side of the map. Eventually Garen succumbed to reason and we were able to pull off a victory.
If one team-member feels like trolling and losing you the game, they can do it easily and there is nothing you can do about it. However, as long as everyone is willing to listen, it is much easier to direct 2 people than 4.