About the complaints about the "simple optics" design, the Kenko 1.4x MC4 DGX somehow seems to be on par or better than the much more complex and much more expensive Canon 1.4x III. There are several reports suggesting this so it looks to be quite objective.
So it seems like the difference between the Kenko and Canon is mostly compatibility, sealing, potential optimization for high end lenses and build, traded for price and size. The actual number of elements seems to be of lower importance and actual element design of higher importance.
Traumflieger: Telekonverter-Test
So even if they take the base design from something like the Kenko DGX 1.4x, it should perform at Canon level.