How reliable is Grover Furr? The short answer is not at all. Furr is a rank Stalinist apologist and not a professional historian. While the latter is not an automatic disqualification, his books and articles twists evidence and interpretations to suit an agenda of rehabilitating Stalin, much as David Irving selectively read archival evidence to rehabilitate Hitler. As such, he is usually banded about by the folks over a r/Communism as proof that the glorious Soviet experiment was never tainted by such sundry details like Gulags, mass murder, political purges, or invasions of neutral territory. /u/International_KB went into a delicious takedown of Furr here and of the wider attempt to rehabilitate Stalin on badhistory here.

You can search JSTOR or Google Scholar--there are no scholarly reviews of Furr's work. That's because he's a confirmed ideologue publishing outlandish claims outside of his field that totally contradict a massive body of established scholarship. It's the same reason why climate scientists don't spend time writing scholarly takedowns of every climate change denier who can get a booklet out.