Train vs car to La Defense Arena

Headed to Paris for one of the Taylor Swift shows and will be meeting up with a large group of friends at the show. I was planning on taking either RER A or the 1 to La Defense. Some of the other people in the group are proposing getting private cars or "party bus" to drive from Central Paris to La Defense arena. My instinct is that sounds like a terrible idea given the potential for traffic and they may be underestimating how long that could really take, and they could wind up arriving a lot later than they intended. Is my concern overblown?