As other reviewers pointed out, there's just not enough information on Rothstein to convincingly portray him as a crime lord, yet his biographers (including this one) consistently attempt to do so.
Here's a slightly different take: Is it just -possible- that maybe Rothstein wasn't a major player, just a gambler and hanger-on who happened to know a bunch of people who were or became notorious?
It's at least as likely as the viewpoint pushed in this book, but I guess that second hypothesis wouldn't sell a whole lot of copies.