For all their flaws, they achieved something that was truly progressive in a society that not too long before was backward and full of violence. It was a mark of progress for humanity. We can and should accept all that and still admit its flaws and mistakes.
Do not let anyone erase that achievement. Do not let anyone convince you that the Soviet Union was a defective system that no longer has any relevance to our society today. Do not let anyone rewrite history. You’ll find yourself fighting an even worse uphill battle if you concede on these grounds.