I've seen him being banned quite often during the pro games but he doesn't seem that strong when compared to roll face champs such as Sett or Darius. Why would they ban him? Is he even viable in solo q or just in pro play?

in pro plays, he can easily turn fights by joining them using his ult.
his opponent on top has no choice but to either TP or watch his team getting shat on.
after that fight, panth can tp back to lane and keep farming / take turret which allows him to easily get ahead of his opponent laner that's stuck on other side of the map after using tp.

his counter in both soloQ and proplays would be Shen, but this meta doesn't seem to go in Shen's favour

i do very very well vs panthy with teemo and urgot