In conclusion: pretty cool Pantheon rework. All Riot did was estrange old Pantheon mains while introducing him to the Top "frustrating to play against" list.
(1) he was only higher in Korea relative to other regions
(2) This was October 2019, around worlds. He was overtuned and 100% banned. I attribute hist higher frustration rating there to Korea being more sensitive to pro play.
(3) frustration isn't the only thing that speaks to the sucesses/misses of a rework.