Sling has one of the slowest projectiles of any artillery unit and is one of the slowest-moving artillery units. It's generally only effective against buildings. The unit description says it's effective against mobile units, but the stats say otherwise.
Against LLT/Stardust Ronin is a better choice, because it also does skirmisher things.
Against HLT most players are likely to build a Phantom instead, because it can also burst down many high threat mobile units you're likely to see mid-game and later.
Against heavily shielded porc Sling might be the best choice, because this can be a case where DPS is better than burst for artillery. Since shielded porc is not seen in competitive play, Sling is not considered a competitive unit.
I would say that Sling has a place, it's just not a place anybody cares about on the ladder. Also the unit description is misleading.