He is simply to strong.
He had a short down adter the trinity changes till people adjusted their builts, but he always was (to)strong.
The main issue is his ult, it's just the single most powerfull abillity in the game.
It has plenty of usecases, can sinpe low health targets on global range without offereing a chance to dodge and deals absurd amounts of damage in teamfights.
at level 6 with thunderlords, the first wave deals instatnly 210 aoe damage, followed up by up to 450 aditional damage while slowing.
For comparison, Annies tibers has 150 damage at that point, so the first wave that comes instantly on global range and can't be flashed out if it's centered directly on top of you deals more damage than annies tibbers(or about the same if we asume she has some solid ap at that point)
Aditionally, the ult can deal up to 600damage(+60 from thunderlord), which equals a full channled nunu ult.
As a result, he can secure kills after close trades elsewhere on the map, he can clear any wave to save a tower, or he can win a dragonfight for his team without ever leaving his lane.
And later on, it just gets more absurd.
at level 11 with deaths daughter upgrade, the first wave deals 210(+110+ from TL) magic damage and 210 truedamage(granted, the truedamage is atleast dodgeable), followed by up to 630 aditional magic damage.
At lvl 16 with fire at will+deaths daughter, the first wave hit's for 270(+160+ from TL) magic, 270 truedamage and up to 1350 aditional damage, adding up to a potential of 1620(+ 160+ from TL)magic damage+270 truedamage.
On a huge aoe, with slow(and speed buff for allies once he has raise moral) on global range
His ult seriously needs a nerf, either to it'S overall damage, or to it's reliabillity, and certainly to it's waveclear.