Sam Heller is quite correct. It is a myth that the FSA ever was or ever had a prospect of being an essential counter-terrorist force. He is also correct in saying others performed better. But you can drive a truck through what he infers from these undeniable truths.
First, the FSA isn't an essential counter-terrorism force because no Syrian force is essential. That's because almost any Syrian force will likely get about the same results if adequately supported. It is either myopic or disingenuous to hold up the superior success of the Kurdish SDF as some significant fact about the relative capabilities of the Kurds versus the rebels. The rebels never had anything remotely resembling the US air support, special forces, intel, and equipment lavished on the SDF. Perhaps that's why, in Heller's world, the SDF seems more 'motivated' to fight ISIS than the rebels. The most he's entitled to say is that we haven't any idea whether or not the FSA would, comparably supported, have done as well.
The only anti-ISIS elements that can be considered essential are Iran's regular and irregular forces. After all, the West wouldn't dream of putting significant numbers of ground troops at risk; that wouldn't go down well with the voters. No other regional power offers anything like the resources Iran commits. To talk about who's essential without acknowledging this plain fact displays a will to distort the region's realities.
Heller's amplifies his righteous indignation by attempting to outbid other analysts in the who's-freaking-out-more-about-terrorism sweepstakes. Yes, Jabhat al Nusra used to cooperate with ISIS. Yes, the FSA did a bit too. Yes, all rebels at some point cooperated with Nusra, & probably will again. However the issue he apparently tries to address is whether the rebels, not only in the past but today, are a credible anti-ISIS force. Are they?
Well, nothing changes your mind about people like them constantly trying to wipe you out. That's what ISIS tries to do to the rebels, with some success. So any rebel groups - if, as Heller seems to say, reliability is an issue - are entirely reliable ISIS opponents. The US could shower them with weapons and air support and no, they wouldn't suddenly switch sides and fight for the Caliphate. Does this really need saying? If Heller is worried that the rebels would use this stuff to fight Assad, he needs to tell us why he thinks it would be a shame that someone, at least, opposed a murderer orders of magnitude worse than ISIS.
Lastly, Heller follows the analysts he likes to dump on by suggesting that the rebels are unreliable counter-terrorist forces, not just because (contrary to fact) they are soft on ISIS, but also because they are soft on Nusra. Here he sinks low. He counts Nusra as terrorist because, five years ago, they set off bombs in Assad-controlled areas. Every party in the region has knowingly killed civilians at some point. Since US air attacks are conducted with the certain knowledge that many civilians will die, it's fair to say that, after Assad & Company, no one does this more than the US. But the plain fact is that, for the past two years at least, Nusra has caused as few or fewer civilian casualties than anyone else. There is also, despite claims to the contrary, no credible basis to warnings that they plan to attack the West. So to call Nusra terrorist is to place one squarely in Humpty-Dumpty's camp: "a word means anything I want it to mean."
At the heart of Heller's 'anger' lies a hatred, not of terrorism, but of Nusra's extreme social
conservatism. You can hate this all you want, but someone posing as a harsh realist should acknowledge that Nusra's attitudes are shared by a large portion of Syria's population. The record of militant opposition to the Assads, going back to the 1980s, strongly suggests that the choice in Syria has always been between Assad and 'radical' Islamists: indeed Heller's claims support that view. No matter how distasteful that choice, the scale of Assad's atrocities dictate a preference for the Islamist alternative. The 'caution' and 'honesty' that drives analysts to cry for Nusra's blood is - if we're being realists - nothing more than de facto support for a mass murderer.