In theory, the '80s multichannel - phased array based SAM systems are less vulnerable.
From what I've heard, 30N6 firing radar of S-300 has very noticeable backlobes (signal from emitter's lens is reflected from rear ends of phase shifters, which are large compared to their aperture for radar signal, and antenna frame), so I bet HARM would be able to home on it from behind. The same applies to S-300V. I guess, that is the reason why newer radars, such as that of Korean KM-SAM, have solid shroud around the radar's rear.
This is simply a general disadvantage of the space feeded, phase shifter antenna systems.
The same applies for the Patriot also.
(Patriot with shroud) 
The main idea is, that if you engage a 2 plane F-16CJ formation, and both Weasels are launching a HARM (half of their total war load for the mission), you can engage parallel the two plane and two incoming missiles, while you still have 2 firing channels free.
Both Patriot and S-300 has capabilities to auto lock on incoming ARM's.
