Sen no sen has a devastating effect – when it works. The reason is that the forward momentum of the attacker adds to the forward movement of the defender. Both movements combined create a level of energy that exceeds the a single punch. While the risk during sen no sen is high the gain for the defender is even higher.
Highest returns emerged when a attack becomes intercepted with a kick. Usually kicks hit targets with way more power. On the other hand, the tend to be slower.
However, good timing and courage can mitigate this disadvantage. How this can work shows Kristian Sundvor in the following video.