First, I have acknowledged that some use the slide step succesfully.
But it is fact that the slide step requires rushing the mechanics.
In a normal situation, when does the hand break occur? When the knee starts to go down (following completion of leg lift).
With the slide step, if the hand break does not occur until the knee starts to go down (extremely quickly, as the knee comes up very little in the slide step), timing will be completely fouled up or the upper body mechanics will be incredibly rushed.
The other alternative is that the break occurs before the knee goes down. The mechanics (timing) are changed and rushed if you will.
Anyone can verify this by either observation or going out & trying it. This isn't philosophy, it is fact.
When it comes to teaching youth pitchers, I am with Grateful. Quick leg lift, no slide step.