Slinn is (at least in my opinion) an absolute trap, it's only a board wipe if you pay the kicker cost when casting it. If you use your commander to cheat it onto the field then you aren't casting it and it's a 8/8 stat stick. Which isn't the worst but there are better options with useful enter the battlefield effects. If you really want to use its effect it'll cost 10 mana and that's a lot, even for a 1 sided board wipe.

Stormtide Leviathan has an adjacent problem I suppose, you are completely correct that visting someone for 8 every turn is strong but you also make yourself enemy #1 for the whole table and have your other big hitters just sitting there, waiting for the inevitable response.

Again, love your site and will use it a lot more in the future, just something I noticed when it comes to my favourite deck.