Last time I paid attention to this, only our B2s and F22s had the penetration capabilities to adequately counter the Russian or Chinese IADS. (I don't know about the F-35). And MC-12s conducting aerial ISR wouldn't stand a chance. SEAD/DEAD is going to be a significant challenge with these highly mobile platforms.
And the only reason for the Russians to have installed a sophisticated IADS network in Syria is as a counter to US/Coalition aircraft, a figurative "I gotcha" just in case you guys are planning anything we don't agree with. They face no threat of aerial attack from ISIS.
The F-35 will be just fine against an advanced IADS system. We may argue about its usefulness for CAS, but that plane is insanely capable in a denied/contested environment.