Sloshers Are Bullshit: Watch as four team members try to hold back a tsunami with the equivalent of a square of paper towel. No holding back, this weapon is going to be a pain in the ASS to deal with. Better get ready for four man Slosher teams during Splat Zone rotations!
Sloshers can take advantage of those tiny boxes that block every shot and still kill you. They can attack high ledges with ease, effectively shutting down sniper perches. It's impossible for snipers to shoot at them because of the ledge blocking shots, but Sloshers have no issue with that. They can Burst Bomb -> Shot combo extremely fast, covering their short range. The 2HKO at every range is actually fairly infuriating. The ink trail starts spreading at the end IIRC, so why not have that be a 3HKO? They have really a fairly fast fire rate, is fairly ink efficient (why), are pretty mobile, and have good range. They laugh at inkwalls.
*Deep breath* THAT SAID, they're really only a problem at tight or cramped spaces with a fair amount of cover. Good luck having them do well at places like Kelp Dome. They're excellent at Walleye because of of the small space. They work really well at the center of Arowana. They're ok at the center islands of Bluefin (me as a Splatling and Ryechu as a Slosher), and they do ok at the Park, capable of shutting down the popular sniper perches, but there's a lot of obstacles, even for them. They're not that great at the top of Saltspray when it's neutral.
Keep in mind that these are observations based off of me using the Splatling and watching streams. Slosher is dangerous at the grates on Kelp, especially that one fork into the bases. I tried killing the Slosher there, but the billboard on the rails blocked my shots and he just flung ink over them. My plays were extremely limited there. Back up, and he has the high ground into my base. Go forward, and I risk getting 2HKO'd if my aim isn't true. Go backwards and he can turn into my base and wreak havoc as I fail at shooting him down from the grates because of the various obstacles. The only viable option in that scenario if I want to kill him is to actually toss an inkwall before the turn into my base and charge up with what little ink I have and hope I don't miss.
The Bluefin islands give a different issue. You can't aim up as a Splatling to get them from above because of the bridge, the high perches can't hit them if they're hugging the center wall, inkwall approaches are useless because lol Slosher toss, etc. I believe Splatling vs Slosher at that spot is actually mostly a stalemate because the Slosher risks getting shot by the Splatling fire.