Mechanical challenges - not so much, they can be solved as many existing aeroplanes demonstrate. Flaps and slates have slightly different functions, as shown in the graph below, from this answer.
- Trailing edge devices increase lift at a given Angle of Attack (AoA), which is desirable - but they reduce the critical AoA.
- Leading edge devices increase critical (stall) AoA.
Particularly aeroplanes with a high subsonic cruise speed have a large disparity in $C_L$ requirements: low $C_L$ in cruise due to the large airspeed, high required $C_L$ during approach and landing. Landing is the critical phase, most high subsonic jets do require the additional $C_L$ and AoA safety margin that the leading edge provides.
Again, mechanical complexity is not really the driver here, aerospace engineers are usually pretty competent at solving complex problems 🙂. The triple slotted trailing edge flap on the Boeing 727 was mechanically pretty complicated...
---Edit---
Please note that the leading edge Krüger Flap functions the same as other leading edge devices, as for instance cited in my go-to book on aeroplane design, page 256:
KRUEGER FLAPS perform in the same way as slats, but they are thinner and more suitable for installation on thin wings. Krueger flaps are often used on the inboard part of wings, in combination with outboard slats, to obtain positive longitudinal stability in the stall.
So it seems that the Krüger flaps are called flaps because they flap outwards when deployed, not because they function identically to trailing edge devices. Also, the above Torenbeek citation identifies another function of the Krüger flap: it creates a nose-up moment, partially compensating for the nose-down moment of the trailing edge flaps.