Just chiming in here, as I also think the sling could use some sort of tweaking. I picked it and the longsword as my starting build along with the striding boots. I played through the Grim Coast realm and I don't think I ever used the sling unless I literally had nothing else to do. There was just never a situation where using it would have been strategically better than using the longsword.
I got my ass handed to me so many times during that realm (I think the rune lord took me like 6-7 attempts). I first chalked it up to me being new to the game and not fully grasping the rules at that time, which was certainly true to an extent. But when I moved on to the next realm, this time using a longsword and shield, I found that I was doing WAY better in combat!
Perhaps that simply speaks to the necessity of having a shield, or at least some sort of weapon that provides Block, rather than the sling's weakness though.