You can't just fight minato he is a legend, tobi has very little morals, maybe due to what he experienced in his life, but not sure if I would call him a coward.
He is scared of rinnegan users thats for sure He didnt dare take Nagatos Rinnegan while he was alive+killed konan as soon as he died and it was stated Pain was stronger then himalso when Madara was revealed in the coffin he nearly wet his pants.
Naruto seems to be giving him a similar sort of effect now