Well, his only two losses are to Chri Weidman and Costa Phillipou. Weidman TKO'd him in the first, while Costa beat him by decision.
That tells me that yes; his ground game is (or at least was at one point) suspect,
and no; getting hit won't make him panic. Phillipou is a guy who will put hands on you early and often, so if Hall was gonna fold, it would have happened against Costa. I'll see if I can find the videos so we can see ow things went down.
With that said, I wouldn't be shocked if he falls flat in the UFC. Dana has already fogotten more about MMA than I will ever know, but evaluating talent on TUF seems to be a weakness of his. He is notorious for mistaking top-level talent for scrubs (Rashad Evans) and thinking mediocre fighters (CB Dolloway) are future stars.
I would honestly not be shocked to see Kelvin finish Hall. I'm finished with doubting Kelvin.