First, Syria has no s300s deployed. Second, no antiaircraft system is 100% invulnerable.
And there is no evidence to prove if ECM wrecks Russian stuff more effectively than say Chinese, German or American stuff.
As for a Soviet junk myth it is 100% related to liberalism. It is based on careful misinterpretation of Arab-Israely and Gulf War facts to make up an idea of total inferiority of enemy weapons. It is all about to convince public into thinking that military doesn’t need any improvement.
The main problem there are few facts to really back that myth because both Arab-Israely and Gulf War were more about superior strategy, tactics and training than about equipment. As for a Gulf War it was nothing but shooting a fish in a barrel. Iraqi air force and air defences were stuffed to withheld Iranian attacks. They had no chance against a thousands aircraft coalition force and still Iraq killed about twice as many coalition aircraft in actual combat than coalition shot down Iraqi planes.
If we are still talking on equipment it also worth mentioning that not only poorly trained Arab forces used not just Soviet equipment but a 20 to 40 yo Soviet equipment or locally-made counterfeit copies. 1980s Abrams vs 1950s T-55 is a bad example of inferiority of contemporary Russian technology. Here comes a relative success of jamming against derelict and tiny air defences.
That same Soviet weapons worked pretty well for competent forces against legacy or newer American stuff manned by incompetent adversaries (see Iran-Iraq, India-Pakistan).
The reality is no one knows how newer antiaircraft systems are working being deployed properly and manned with trained personnel.
Last time a sufficient systems contemporary to facing them American assets were tested in Vietnam. There were ECMs at the time as well, but USAF&USN lost about 3000 jets anyway.
ECMs has improved for decades but so did SAMs. I don’t think there are airmen lining to check it out.
Honestly, do you work for Almaz?