As an RB2, he was everything you could want. Defined strengths that mesh with the run game philosophy. He comes with limitations but you don't have to expose him to a ton of variety if he's just a bit-part player. As you said, once they had to open up the variety of his workload, he typically suffered.
His 2016 season wasn't just good, it was goddamn historic - https://www.numberfire.com/nfl/news/12615/mike-gillislee-had-the-most-efficient-rushing-season-of-the-century