Biggest difference is that Lamar Jackson threw for 3390 yards (and 30 TD's)...and oh, yeah, also ran for 1538 and 21 more TD's.
JT Barrett threw for just 2428 yards and 24...and ran for 847 and 9. What's particularly interesting to me is his yards per attempt...which is just 7.02 yards. That's low. That 7.02 YPA puts Barrett down at #77 nationally, on the low end of college QB's. It tells you he isn't throwing deep much and isn't connecting when he tries it.
In contrast Deshaun Watson threw for 3914 yards, 37 TD's...and added 529 and 6 TD's with his legs as well. He also averages 8.04 yards per attempt, a full yard better per attempt than Barrett.