That’s the takeaway from a new working paper by Brian Haggerty, a doctoral student at MIT. His conclusion:

The United States and its NATO allies no doubt possess the capabilities required to achieve some measure of air superiority over northwest Syria and to maintain patrols over population centers to defend them from some incursions by Syrian forces equipped with heavy weapons. But as this analysis shows, an intervention to establish only three safe havens, in Homs, Hama, and Idlib, linked to each other and to the Turkish border via a humanitarian corridor, would be a substantial military undertaking. Given Syria’s air defense capabilities, the ubiquity of its tanks, artillery, rockets, and mortars, and tens of thousands of al-Assad-regime allies willing to carry out acts of repression, it does not require any heroic assumptions to suggest that such an intervention would require greater resources, face greater risks, and have a lower probability of success, than any of NATO’s previous air campaigns in response to humanitarian crises in Bosnia, Kosovo, or Libya.

This conclusion is derived from two major considerations. First, Syria possesses
an air defense system with enough mobile surface-to-air missile systems that any attempt
to defend safe havens from the air would require a major, sustained suppression effort for
the duration of the campaign. This would not simply require a large expenditure of
resources up front in order to degrade Syria’s integrated air defense system (although
such a large expenditure would indeed be required); Syria’s strategic air defenses could
likely be degraded or destroyed relatively quickly. The problem is that Syria would still
possess large numbers of tactical mobile SAMs (some quite advanced) that the United
States and its NATO allies have historically had little success in destroying outright when
adversaries have failed to be anything less than cooperative…..

Second, the al-Assad regime still maintains enough strength on the ground,
whether elite elements of the Syrian Army, the thousands employed by its security and
intelligence services, or its shabiha militias, to ensure that determined allies of the regime
could still carry out attacks against civilians that would perpetuate Syria’s humanitarian
crisis. Even if NATO were willing to deploy enough strike aircraft to maintain 24-hour
coverage over safe havens in the northwest capable of engaging significant numbers of
Syrian fielded forces within short periods of time, it would still have only limited ability
to detect and identify hostile elements from the air. Crews flying strike coordination and
reconnaissance missions would have little ability to prevent the infiltration of Syrian
forces carrying small arms and capable of carrying out many of the repressive tactics that
have thus far contributed to Syria’s humanitarian crisis (e.g., the massacres at Houla and
Qubeir)….

Thus, despite a decade of advances in ISR technology since NATO operations
over Kosovo, the problem of emerging target detection and identification would still pose
a major challenge for NATO air forces without help from boots on the ground, and was
so even in the relatively permissive airspace over Libya. The “true worth” of air power, then, still appears largely to reside in its effectiveness when combined with highly trained
and capable ground forces. To hope for air power as a “low-risk” alternative to the use of
ground forces in Syria or future humanitarian interventions would thus be to
misunderstand the basis for air power’s relative success to date. 

You should read the whole thing, as Haggerty provides a nuanced and thoughtful analysis. My sense is that he’s basically right. Degrading and suppressing Syrian air defenses would be a major undertaking; NATO would need a significant ground presence of some sort to leverage its air assets against Syrian army operations. Indeed, the best argument in favor of an “easy” intervention comes down to the claim that NATO’s airpower would deter the Syrian military. But that didn’t happen in Libya, a country with far less robust military capabilities and well-trained conventional forces.