Did you ask yourself for how long Obito has been fighting against thousands of people and controlling different creatures/bodies in the meantime?
Of course now that he was about to "die" he can't handle the strongest monster, but if he is in a good shape, there's no reason for him to not accomplish it.
Besides both Madara and Obito wanted to complete IT all alone which means they can handle it if they try harder.
Kishi trolled Juubi so everything like taking control over it even if you're dead man walking can be expected.
Of course now that he was about to "die" he can't handle the strongest monster, but if he is in a good shape, there's no reason for him to not accomplish it.
Besides both Madara and Obito wanted to complete IT all alone which means they can handle it if they try harder.
Kishi trolled Juubi so everything like taking control over it even if you're dead man walking can be expected.