Having now googled the two more recent reviews, they may be characterized as the Arabists vs. the Syriac scholars. The two reviewers in Hugoye are specialists in Syriac (Luxenberg's field). The two reviews in the Journal of Qur'an studies are both from Arabists (and one of them quite eminent, but with a bibliographical tendency towards more recent literature.
Dunno. I'm certainly not qualified to comment on substance, but the disciplinary breakdown is not encouraging -- it's a sad parallel to, for instance, the division between archaeologists and classicists talking about Homer and Mycenaean Greece before 1900 -- so oftren they talked PAST each other.
The field is terribly tendentious anyway. Luxenberg will have to stand or fall not on his book alone, but on the results others get by applying his method (yes, historical linguistics and philology are a tad more scientific than many aspects of the humane letters).
I agree that philology is more rigorous than, say, literary criticism, but I'm not sure I share what I think is your optimism. I'm not sure philology is something that Arabists are used to. While it's been unremarkable to apply it to ancient texts since the days of Isaac Casaubon, and to the Bible since the 19th Century, it seems to me that students of classical Arabic rather resent the intrusion of this Western hegemonic strategy (!) into their field. I think they follow Edward Said's rather hostile takedown of Bernard Lewis for looking merely at the roots of Arabic words.