A welded joint carries load through the cross-section of metal actually fused together, so the size of that area governs the force the joint can transmit before failing.

An edge joint is made by laying two plates flat with their edges flush and running the weld only along those meeting edges, so the fusion is confined to the thin thickness of the plate edges.

Because only these narrow edges are melted and joined, the resulting weld area is small and cannot spread stress over much material, which keeps the load-carrying strength of the joint low.

When a component must bear heavy, shock or repeated (high) loads, this small weld area quickly reaches its limit and the joint cracks or tears, so the edge joint is unsuitable for such service.

The options claiming a large weld area with high strength, or wide accessibility with significant reinforcement, describe the opposite of an edge joint, whose flush edges give little metal and little reinforcement.

High ductility and flexibility is also not the reason for its weakness; the real problem is the lack of load-bearing cross-section, not excess flexibility.

Hence, the answer is Limited weld area and reduced joint strength.