Various Repub members of the "national security community" have delivered a blast at Trump (h/t). A glance at the list of signatories reveals some big neocon names (though not all are neocons). This open letter is going to have no effect on Repub primary voters. As to the substance, many (not all) of the points are, imo, valid, but that's partly because they're pitched at a high level of generality. It's mildly amusing, in a black-humor sort of way, to see so many supporters of the G.W. Bush admin expressing grave concern about Trump's remarks on torture.
ETA: NYT notes some people who didn't sign the open letter.