It is not so much Roberts is a chickenshit as it is that he is out of his league when it comes to the brain power required in the first place; and compared to his peers on the court(except Ginsburg whose brain is smaller than the others--just look. lol.).
Making matters worse is he is chief justice. It is one thing to not be the smartest person on the BOD but you may not want to be the Chairman of the Board. You might try to make up for it if you know what I mean.
The final, and new, problem/challenge for him as the intellectual lightweight is that he is now the swing vote. If he could comprehend the weighty issues like Gorsuch and Thomas he would not be the swing vote. It would be Alito, Breyer, or even Kagan.