Pakistan invests in chinese air -defenses- gets steamrolled by india. Iran buys chinese air-defenses- gets steamrolled by Israel and the Us. Russia claimed the s400 was all the rage- and its going nowhere in ukraine. If propaganda claims where a currency, could you buy anything with all this?
The S400 and the Patriot etc all suffer from the same problem: They only deny the enemy high elevation flight.
Russian doctrine has been based around big, fast things going high. NATO doctrine has been about smaller, slower things going very low. Going low leaves you very vulnerable if you get too close to a defender, but there's no way there are defenders everywhere.
An extreme example of the problem was the Moskova--big, fast missiles that couldn't see something coming in just above wavetop height. There were only two launchers that had any possibility of engaging and only time for one launch cycle--and that probably only if they already had a bird on the rails. (Exposed to the elements, rather than safe in the magazine.)
air defense is much more complicated and difficult to build.
Iranian cheap drones/cruise missiles are efficient from another hand.
Pakistan and Iran's imported missiles from China have worked as-advertised. To say nothing of the PL-15's recent success, China's land attack missiles have been a serious threat ever since the Silkworm ASCM hit the export market.