Your quote mining to downplay his criticism of Farber aside, why don't you try reading what he said:
Here, Farber assumes much of what was at issue during Lincoln's presidency: that the Union was simply a territorial unit, not a group of sovereign states voluntarily joined; that the Constitution was what Lincoln said it was, not what his opponents to the south held it to be. Farber's assumptions on these scores shape most of the rest of his book. In sum, LINCOLN'S CONSTITUTION is a partisan work, more a lawyer's brief for the Lincoln administration to be argued before a contemporary American court or group of academics than an exercise in historiography. It is none the less interesting for that.
In case you didn't know, that's a polite way of saying it was an unscholarly hack job posing as history.
No, the reviewer may not like some of the sources that were used, but the work nevertheless is worth reading.
As for being pro-southern, I read some of his opinions and they were clearly pro-Southern.
Nevertheless, he still does not represent a community of scholars