Not, it's not hard, but what's the value of the time data if you push the button some indeterminate amount of time before you actually start, and some indeterminate amount of time after you stop? You can do just as well, if not better, just using the pace clock. At least with the pace clock you can sync your start somewhat accurately. Even the swim watches that supposedly detect starts and stops leave a lot to be desired when it comes to accuracy. If you don't finish the rep by touching the wall with the watch hand before the other hand, the time data is garbage. Even if you do touch with the watch hand, it's suspect because the start detection logic is far from perfect.
That's why I use a finger stopwatch when I actually want accurate rep times. I can easily sync the start with my push-off, and the stop with my touching of the wall with either hand without altering my technique.
"They're made of latex, not nitroglycerin"