I know that I've seen very unfavorable opinions of his non-classical works such as "Carnage and Culture", which are very Huntington/"Clash of Civilization"-esque, but he first made a name for himself dealing with classical warfare, as I understand it, so is he better respected when he isn't straying outside his field?
With ancient warfare he knows his way around the material, and he is taken reasonably seriously in some respects. But right from the word go, his work was always intensely political, focused on celebrating the West as "special", and on celebrating ancient Greek soldiers as independent small-holders. What he means by this is, effectively, American ranchers (he is one himself) transplanted into ancient Greece. This ideological message was blatantly obvious starting all the way back with Warfare and Agriculture in Classical Greece in the early 80s. His ideological narrative underpins pretty much everything he writes, enough so that it undermines his early work as much as his more recent stuff.
Having said that, when he started his work on ancient history, writing about American ranchers transplanted into ancient Greece, his angle was a novel one for anglophone ancient historians. Looking at the relationship between warfare and ecology and, in some later books, ancient soldiers' experience of ancient warfare, was pretty innovative. So his books of that era are seminal, even if they're thoroughly slanted by his politics. As a result, he was being honoured within the field up into the 1990s for the stark immediacy of his message.
Later work made some less agreeable sides of his politics more obvious. His book Who Killed Homer? was an attack on the academy for, basically, not celebrating the Western tradition as the be-all and end-all of human civilisation. (This included things like celebrating the intensely disenfranchised position that women held in ancient Athenian society.) He's attacked Jared Diamond... not for his methodological flaws (which you can read about in many threads in this subreddit), but because Diamond didn't give enough credit to Westerners.
But particularly in his earlier work, he was reasonably conscientious about data, even if he sometimes interpreted it violently. So when it comes to things like the pure practicalities of warfare, he's no authority on the subject, but he is taken about as seriously as any moderately respectable scholar. He's no Niall Ferguson, in other words -- but then again he wouldn't want to be (British conservatism isn't nearly agrarian enough for his tastes).