As a user of overhead reels and imp clips I find non-release is an issue at times. If there is any pressure on the line (such as too much spool brake, or severe cross-wind) when the rig hits the water, the line pressure is enough to overcome the water pressure on the clip paddle. Twenty minutes later you wind in to find the bait gone but the rig still clipped.
The other problem with imps is load. A large or heavy bait, such as half a large pilchard will not stay clipped and will unclip in flight.
Even so, they are still my choice of bait clip. Ease of use overcomes those occasional problems in my book