McCaffrey is a poor (white) man's version of Marshall Faulk. Faulk barely ever carried it over 300 times in a season but he also caught about 70 balls a year.
McCaffrey won't be as good, nor do I think he can handle 250 carries a year like Faulk, but I do think it's fair to call him a poor man's version.
As for Fournette, I think he's overrated because of how aesthetically impressive he is. People love to see the tall, lean running backs who gallop with speed and power, but I think in some cases that body type and running style can lead to injuries as it does with Fournette. Of course Peterson also had injury issues in college and got over them for the most part in the NFL, so Fournette could also, but I'd take a squattier, lower center of gravity RB like Elliott 10 times out of 10 over the tall, lean guys like Fournette.