As a general rule, great fighters make horrible trainers. And it doesn't just apply to boxers, it's all sports really.
Great fighters just can't relate to working with fighters who don't have their natural ability.
I think Pernell Whitaker is going to be a pretty bad trainer. During his career, Whitaker never even watched tape of his opponents. He would just show up in the ring and "figure his opponent out" during the first few rounds. 99% of fighters can't do that and get away with it.
If you look at the top trainers, generally they were journeymen fighters, guys who couldn't get by with just talent.
Freddie Roach, George Benton (Whitaker and Holyfield's old trainer), Floyd Mayweather Sr. are examples of this.