This has to be by far one of the most ignorant and bigoted answers I've ever read on this site, or on this question in particular.

If Islam is as unique as you say it is, why is it that Pakistan is able to constantly hold its ground against India, ever since the independence of both countries? Why Afghanistan beat conventional armies regularly, including the most powerful military in the world? Why Chechnya was such a thorn in Russia?

The answer is much simpler - Arab society is built upon tribes, and the tribal system continues today. Rulers of autocratic countries want loyalists from their allied tribes at the top of the military, and tribes do their best to look after solely their own interests. Never mind that oil wealth has also enabled insane defence spending, which means with the right positioning, some well-placed members of the military could stand to earn millions in bribes and skimming, which makes a career in the military all the more lucrative for the tribes.

Which is why ISIS was so skilled and so much of a threat to the Arab countries - because their motivation was not to enrich themselves, it was purely driven by their radical ideology. The best killers are those who don't give a shit about their own lives.

I think you're probably right, but your counterexamples don't really refute the strongest reading of the thesis. The military establishment has always ultimately held the reins in Pakistan, so the dynamic of civilians avoiding threats by rewarding incompetence isn't there - the threat is present anyway and the rewarding isn't in their power. Afghanistan and Chechnya are confederations of tribal warlords, so of course the armed forces are controlled by competent fighters, it's almost a tautology. You need an example where there's a strong civilian central authority that controls the military effectively, and maintains a highly competent military, and isn't constantly troubled by the threat of coups.