Obito knew imo, that his wood is not strong enough do give kakashi problems... When kakashi gained kamui everybody forgot all his other abilities like speed and good reflexes and speed.
Kakashi tanked and dodged kakuzu's strongest atacks... while they were far more dangerous than obito's wood.
Also, they bijuu-sized fire ball jutsu wouldn't have worked, since obito has to breath deeper for making a larger fire ball, and if he makes such a long oppening to gather enough chakra, he gets decapitated.
I'm not saying obito wasn't handicapped, but WOOD and FIRE wouldn't have worked anyways, since kakashi was WATER and HIDING LIKE A MOLE (OTHER DOTONS) that he didn't use either.
The only handicap obito had is not having kamui and gedo mazo, but wood and fire were at his disposal, he just didn't use them because he knew that couldn't have worked against kakashi
Kakashi tanked and dodged kakuzu's strongest atacks... while they were far more dangerous than obito's wood.
Also, they bijuu-sized fire ball jutsu wouldn't have worked, since obito has to breath deeper for making a larger fire ball, and if he makes such a long oppening to gather enough chakra, he gets decapitated.
I'm not saying obito wasn't handicapped, but WOOD and FIRE wouldn't have worked anyways, since kakashi was WATER and HIDING LIKE A MOLE (OTHER DOTONS) that he didn't use either.
The only handicap obito had is not having kamui and gedo mazo, but wood and fire were at his disposal, he just didn't use them because he knew that couldn't have worked against kakashi