I hear what you're saying and definitely think he's underperformed for his career, given what was expected, but he's an unorthodox all-purpose guy.
In 3 of his 6 seasons he's tallied 69, 73 and 76 receptions. For that kind of usage, I can give a little on the rushing numbers.
Add in that he's excellent in pass protection and the total sum of what Fournette gives you is pretty valuable.