steel types, physics, goats, homestuck, warframe, RWBY, FFXIV, fire emblem to an extent, modding, oh yeah and some animes, esp. anything with giant robots. Please read The Last Girl Scout
He lacks a strong STAB move, just like every physical Ghost type except Marshadow, and he lacks a consistent recovery move, limiting his stall options. Plus his HP are low.
But he is still decent for a run. Smogon is played under specific rules. Not every pokemon can shine there.
Probably just to keep burning physical Mons throughout the game. Pain Split is actually alright for this, given DK's abysmal hp.
Also yeah, we could really use a widespread 80+ power physical ghost move, and a 50-60 power widespread physical poison move for Mons like Grimer to be decent early game.