First of all he is an Uchiha with a (Mangekyou) Sharingan. He has the aptitude to use and disarm genjutsu with ease and to a good extent. However there is another thing that makes him better at this. We know that Obito can use Kamui to send specific parts of his body to the Kamui Dimension. If he sends his brain to the Kamui dimension he could essentially be genjutsu-proof. He can't die from this because he sends like his head to the Kamui Dimension and is perfectly fine. Say speaking of this, can he like send his soul to the Kamui dimension and live forever ... even when his body is destroyed his soul is intact? Whatever, this is why I think theortically, obito is genjutsu proof