Control track editing has two major shortcomings. First, it relies on equipment to maintain an accurate count of thousands of control track pulses. For basically mechanical devices this can be difficult. During editing you are constantly shuttling tapes forward and backward at varying speeds as you find and mark in and out points for edits. If equipment loses count just for a fraction of a second, an edit point will end up being off by a few video frames which will destroy a carefully timed edit.
Savvy editors keep an eye on the digital tape counter as tapes are shuttled back and forth.
If the counter freezes for a split second when the tape is moving, the equipment has momentarily lost an accurate count of control pulses. Since the control track count is kept in the volatile memory of computer chips, when the machine is turned off or reset, all edit decision information is lost. Because of that they do not remain accurate when tapes are changed. T he only way to insure accuracy is when exact locations become permanent information on the tape. When time-code is used exact scene locations remain consistent even when tapes are used in different machines at different times.