Any weapon system’s effectiveness ultimately depends on the users ability to wring every last bit of performance out of it so that a war winning performance can be achieved.
Soviet weapons, and the weapons of the Russian successor state are very well designed and effective for their purpose, judging by most open source reports.
Israel, for example, used a lot of captured Soviet weapons, including tanks and artillery pieces, to excellent effect in its many wars with the Arab states. Similarly, North Vietnam inflicted losses on the US in its long war against American forces, including downing over one dozen massive B-52 strategic bombers with Russian SAMS and AAA.
(Photo Credit: http://vietnamveterannews.com USAF B-52’s on a bombing run)
As to the Arabs their weapons did not fail them in every war. It is true that Israel has won every war so far against the Arabs, but Arab forces have scored against the Israeli’s as well, using Soviet equipment.
For example in the 1973 Yom Kippur war the Egyptians stunned the Israelis by destroying hundreds of their tanks with the deadly new Sagger ATGM’s. The Israeli air force, the IAF, was similarly stunned when Egyptian air defenses, designed by the Soviets and using Soviet weapons such as the lethal SA-6 SAM system, blasted well over 100 Israeli fighter jets and attack aircraft from the sky in short order.
Egyptian forces achieved these impressive initial victories because they analyzed their mistakes, listened to their Soviet advisors, trained hard with their new weapons, and became the most professional of the Arab armies by the time of the Yom Kippur war, barring the possible exception of the well regarded Jordanian forces.
So the problem is not with Soviet equipment but with the forces using them. Well trained forces proved to be effective and powerful opponents, while lavishly equipped but poorly led and trained forces served only as cannon fodder against a capable opponent.
India achieved victories in 4 major wars against Pakistan and 1 border conflict against China (in 1967) using largely Western weapons (British, French, and some American systems as well) in its first war in 1947 and from the 1965 war onwards increasing proportions of Soviet Union then Russian equipment.
India succeeded where the Arabs largely failed (excluding the early success of the Egyptian forces and the always good performance of the small Jordanian forces) because India studied its past wars, understood the opponent they were fighting, bought weapons that were well suited for its operational environment, trained to use these weapons to their full potential, and had disciplined well led troops and competent generalship.
Additionally, India had considerable technical and scientific abilities which enabled them to integrate their weapon systems into their warfighting formations far more quickly than nations with lower technical and scientific starting points.
All of these factors allowed India, fighting hard against a determined and formidable foe who matched them in most combat parameters on the ground and air (at least up until late 1990’s) to achieve a string of victories. These victories showcased both India’s armed forces abilities and the weapon systems they used, ranging from homegrown Indian systems, legacy British systems, new generation Anglo-French, German, and Soviet/Russian systems, and niche offerings from nations such as Poland, Czechoslovakia, Hungary, and others.
So all told, its really no surprise that India won its wars while the Arabs lost most of theirs.