Z axis counterbalancing pros and cons

Hi all,
I'm considering counterbalancing the Z axis carriage for my router to offset the spindle weight and stay with a small drive motor. The plan is to run a cord from the top of the carriage over a ball bearing unit located at the frame top which suspends a weight dropped down the back side. What, if anything, am I overlooking? Would anyone have any suggestions or gotchas to contribute?
TIA
Denis