Short answer: Soviet equipment was quite good, but their commanders stank. They got lucky that a few brilliant commanders survived the purges and were given a free hand at Khalkhin Gol.
The problems of the Soviet army in WWII are more ones of leadership than material. They had the men, the equipment, and the tactics, but they had few good leaders to take advantage of them.
Prior to the purges, the Soviet military was feared. Their military was enormous, equipment the interwar standard range of awful to revolutionary, and their tactics excellent. In the 30s, Soviet doctrine followed Deep Battle which is remarkably similar to Blitzkrieg.
Then the purges happened. Most of the top level leadership was killed or banished to obscure posts. Those who remained or were promoted knew more about political reliability than military tactics. Yet pieces still remained of the interwar brilliance.
One of these pieces were the Soviet leaders who were brought in to escalate and win the growing border skirmish, one was Georgy Zhukov who would survive all the purges and go on to win the war for the Soviets. The Soviets built up their strength (500 tanks and 800 aircraft to the Japanese 135 tanks and 250 aircraft), defended against attacks, and then executed a classic double envelopment using concentrated armor to break into the Japanese rear area.
In contrast The Winter War with Finland was planned and fought by leaders who showed little imagination than to bludgeon the Finns. The Soviets relied on frontal assaults, used poorly trained troops, and brought tons of useless anti-tank equipment which fell into the Finns' hands. It was only after a pause and shakeup of leadership and tactics did the Soviets made headway. However, they did show a logistical brilliance in their ability to move and supply 21 divisions (far more than the Finns expected possible) all across the inhospitable Finnish border. The Soviets won their objectives, but the Finns also won in keeping their nation intact. Both took horrific casualties.
Khalkhin Gol was a success, and so would not have made the Soviet high command reflect. It was the humiliation of the Winter War which caused the Soviets to begin reforms, but it didn't happen fast enough to be ready for the German invasion.