While it's not often that you can spot a problem with a god before they even hit the live servers, looking at Cernunnos' numbers on his abilities I already spotted one which is much too powerful.
I of course am not making this claim in a vacuum, I have maths to prove that the protection-shred on Cernunnos' Shifter of Seasons ability is very overtuned. In fact, it is too powerful for the same reason they nerfed Season 2 Executioner. For those who don't remember, Season 2 Executioner used to reduce Physical protections by 8% and 6 flat for 3 stacks. Essentially making it 24% plus 18 flat Physical protection reduction. This was too good because it worked well against tanky and squishy targets alike.
With Autumn Decay, Cernunnos can buy Season 4 Executioner and have a more powerful version of Season 2 Executioner right in his kit. With both Autumn Decay and The Executioner, he has a 36% and 25 flat Physical protection reduction. Here's the math to show how powerful that is:
Autumn Decay + The Executioner vs. Squishy Target
Lvl. 20 Ra = 60 Physical Protections
Actual Defense = (60 × (1 – 36%) – 25) × (1-0%) – 0
Actual Defense = 13.4 Physical protections
Effective Physical protection reduction: 77.7%
As a support main, I know to aim for about 250 Physical protections, which is about where you really start hitting the diminishing returns cap for protections.
Autumn Decay + The Executioner vs. Tanky Target
Lvl. 20 Guardian ≃ 250 Physical Protections
Actual Defense = (250 × (1 – 36%) – 25) × (1-0%) – 0
Actual Defense = 135 Physical Protections
Effective Physical protection reduction: 46%
As you can see, the sheer power of Cernunnos' Physical protection reduction is not to be underestimated. As a Diamond Nemesis, I can tell you that reducing protections is incredibly powerful in a team game because it makes the target more vulnerable to all damage sources of that type. While Nemesis' ult eventually wears off, Cernunnos can keep this debuff active perpetually with basic attacks.
TL;DR – Cernunnos' Autumn Decay plus current Executioner gives him 36% physical protection reduction plus 25 flat physical protection reduction. This is more powerful than season 2 Executioner with 24% physical protection reduction plus 18 flat physical protection reduction. That item that was "too good" because it was good against tanks and squishies alike so it needed to be nerfed. Ergo, we can deduce that Autumn Decay + The Executioner is "too good."
Say for sake of argument you bought the max Physical protections possible to stop Cernunnos from shredding it with his 1 and The Executioner.
Actual Defense = (325 × (1 – 36%) – 25) × (1-0%) – 0
Actual Defense = 183 Physical Protections
Effective Physical protection reduction: 43.7%
Meaning even if you got the maximum Physical protections possible at 325 Physical protections, vs. 250 Physical protections you've only reduced the protection-shred of Autumn Decay + The Executioner by 2.3%. You bought 75 Physical protections, and only got a 2.3% improvement.