I would liken Insanity burst to Frost mage burst (pre-nerf): reliable heavy burst every 30s, yet still stoppable. In contrast to Frost mage burst, Insanity burst is much easier to pull off, deals more damage, and happens over a shorter period of time. Insanity requires 6-7s free cast. SW: D and DI finishers do not require true 'free cast'. Compare it to Shatter combo (pre-nerf) which requires Frost Bomb, (Freeze), Frostbolt, Fireblast, FFB, Deep Freeze + IL x3 which is 7 GCDs (~9s window).