That was a limitation of the initial implementation in UE4.14, not the technique itself. They iterated on it in UE4.15: https://www.unrealengine.com/blog/unreal-engine-4-15-release...

> Forward renderer now supports shadowing from movable lights and light functions.

> Only 4 shadow casting movable or stationary lights can overlap at any point in space, otherwise the movable lights will lose their shadows and an on-screen message will be displayed.