First of all, the skill force on the skill is misleading. The reason behind this is that it scales exceptionally well with level, and a maxed rolling thunder 7 has triple the sf that it does at level 1.
Secondly, the skill force of that skill corresponds to the damage per tick, and, as each rank gains an additional tick, that implies that each rank scales up incredibly, with 7 ticks of 2xx skill force at max level at rank 7.
The range also grows with each tick, so yeah. But in the first place, that's not even an issue when you use it in combination with tidal wave. By that, I mean you can use rolling thunder inside of tidal wave's animation, and basically wipe anything inside instantly, assuming that the monsters you're fighting don't use projectile attacks. (Don't use rolling thunder against mobs with projectiles.)
Note that its only good at high levels and high ranks.
So... given a Level 108+ Spear player has +100 +Omen Doom+, with Water Dragon Spear and Ice Blossom maxed, if given that the player can afford to add any Ougi skill at all to the 2 Ougi slots, should Devastation Slash or Rolling Thunder fill the remaining 3rd skill spot?