In my frustration get the strobe effect (in Motion) to work in a commercial plugin I'm working on I decided to publish set of free strobe plugins that are aimed at specific frame rates. They're basically a rigged version of the motion effect strobe, designed to only allow you to choose rates that will produce a smooth cadence.
FCP Legacy did it right by simply letting you choose the hold duration of frames, so there was never really a cadence problem. But the motion strobe effect lets you set the frame rate, which means you can easily choose something like 11fps or 13fps or 9.7fps, giving it an odd cadence to say the least.
These plugs fix that dilemma by doing the math for you with specific plugins for strobing clips in 24p, 30p, and 60p sequences.
They also fix the bug where the first few seconds that are strobed in FCP X often have unreliable results with other published versions of strobe. Sometimes the video would even go backward. I found the trick to fix that little nuisance.
Give them a go. Hopefully they will become part of your toolset.
_______________________________________________________________________ http://BretFX.com Plugins & Templates for FCP X Editors & Motion Graphics Artists