Simon already beat me to it, but I will add to it any way. There is a broad misconception that "cheap" tuners "slip" and cause poor tuning stability. Simon quite adequately listed the most common actual causes for tuning issues, so I won't repeat that.

IMO the biggest issues with cheap tuners are the machining tolerances, particularly with the internals.
Their rotation can be less smooth and "feel" looser and the turning ratio is generally higher (like 18:1).

I can't give you an actual figure, but the amount of tension it would require to turn even a cheap kit tuner backwards (so-called slipping) by pulling the string, I reckon the string would break or fail before the string post would move.