What I meant by counter is avoiding his skills, every single one of them is either tied to him or to a specific location, the only exception being the E.
Yes, he's not as gimmicky as some of the most gimmicky champs, that doesn't make him non-gimmicky.
Compare all of that to his old kit which was:
-Gain mana on killing stuff
-Deal dmg over time and slow as long as they're near the bird
-AoE root and dmg with slight delay
-DoT with increased dmg taken from his other stuff
-Dmg 3 nearest units and heal for the dmg dealt as long as you have mana, prioritizing champs
Simple, straightforward stuff where it's clear what's going on at all times and how to avoid it. The only straightforward skill in his new kit is the Q, the W has stupid range for some reason like he's Xerath or something, have fun getting sniped by a supposed short ranged tanky mage from under his tower. His E is also finnicky to dodge since it roots in an AoE on the first hit when it returns, how big is that AoE? It might root one of your teammates or it might root you all, unless you know it's range by heart I don't think you'll be able to tell immediately if you're in danger or not, compared to his old root which was extremely clear in its AoE and you could reflexively dodge it if you were paying attention. While his ult is mostly the same the extra damage finisher is also finnicky and obfuscates the potential damage, because the number of fragments is tracked by the rather tiny pips under his HP bar and I'm not even sure if those display if you're on the enemy team.
Tho once again, the question is was this actually needed? I'm saying no because old Swain was a straightforward battlemage with a clear kit that had clear upsides and downsides as well as counters. The new one seems like a hodgepodge mix of some of his old kit and somebody elses shit with little thought put into how it all actually meshes together, both for the player of him as well as the opposing team.