When launching players horizontally with a slime block, the player does not get launched and instead is only pushed 1 block like any normal block. This applies when the player being pushed is in the middle of the block; standing on the (right or left) edge of a block will still launch a player for some reason. To explain this better, if the whole hitbox of the player is hit by the slime block it wont launch the player. If only half the hitbox it hit, then the player DOES get launched.

This does not apply while launching upwards. I have used ice under the player to exaggerate the effect of the horizontal launch but while standing in the middle of the block, it still does not work.