My first thought is that is not terribly unique at first blush. Rod of Lightning has been a D&D Staple for decades, and Rolemaster has that exact spell. You will need to do something which makes it truly your own to start with, and that might suggest good weaknesses.
For a simple 'Call Lightning' rod - the best disadvantage is it needs to be used outdoors, with clouds. But again, that is straight from the book and not the slightest bit unique.
As for power, one can always reduce the voltage, range, rate of use, add random other bolts uncontrolled by the user, etc. But take care there too, otherwise it begins to feel like a wand of wonder if it is too erratic.