This answer!Yes, It takes a Sage to Hurt Obito, and now you have two in the Battlefield. Obito can stop Tail Beast attacks because He is a Tail Beast now as the Juubi and He can Stop Ninjutsu, but Sage Mode is Natural Energy a totally different out of normal world power.
Don't forget Madara was the one who planned all of this and he also planned on becoming the juubi's junchuriki himself and in doing so i think he would learn about his weaknessesHow would he know that, though?