Im russian, i trained both Sambo and Bjj, and traditional JapJJ. The thing is , those coaches who train you in sambo, they know all the most important keys and elements to bjj, also including a lot of judo. After learning sambo i made a transition to JapJJ and then to Bjj. Sambo taught me how to enter and get a better balanced throw positions, knowing how to shift your wieght when you are on your back. JapJJ is very soft art, very simple mooves that require only minimum amount of energy used, submission where mostly from the top positions. And Bjj teached me how to really explode in the right positions, gave me better mobility , and great usage of my hips.
People dont train Bjj in Russia cuz all the true russian coaches say that sambo is the universal grapling art, and it derives the best out of the greates arts.