I think one reason people may have a problem with LE flaps (NOT slat) is that they are divergent (kinda the wrong term here but close enough) under some sort of load.

So if a normal trailing edge flap bobbles a bit in the breeze, the air loads tend to blow it back up, decreasing the load on the servo.

If a leading edge flap bobbles, the breeze tends to blow it farther down, which increases the load on the servo.

I think this is part of the problem people have had. When the LE flaps are up, they gotta STAY up, at high speed. Cause if they go down even a bit, I think it will be bad.

All just my opinion. I don't thing slats have this problem, or if they do it is to a much lesser degree.