Considering at best, Kartana is doing 56% with +2 Smart Strike ( a move that isn't super common on SD Kartana right now), that's not an enormous issue. Drain Punch not only can't miss, it would allow it to heal up on things, something it would otherwise rely on Roost and Leftovers for.Hammer arm ohkos Kartana, something drain punch can’t do.
That's just in terms of vs Kartana, there may be other 'mons it misses OHKO's on I don't know of, feel free to let me know.
With that said, in terms of pros vs cons that I'm aware of, I personally would rather run Drain Punch.