It's the fact that Swain and Brand are being nerfed when it's not their kit or their base stats that are the problem.
How do we know this?
Swain and Brand have never been an issue before, both of them have been considered weak/balanced for a very long time.
The problem?
Deathfire Touch. It's too good on 'Damage Over Time' Abilities. The reason it's too good is because it procs every single time a DoT deals damage, which basically doubles the damage output of that skill.
Why are we worried?
Riot took this same approach to Warwick, Sejuani, Kassadin and now Swain and Brand. An Item/Mastery made a champion a little on the strong side - so instead of nerfing the item first, Riot nerfed the champions first. The Item/Mastery continues to be a problem, so Riot then nerfs the item - but seems to forget they just nerfed champions because of the item, and walla - said champions are now even further in the coffin.