Do you also believe in perpetual motion? Lenses curve different colors of light different amounts. You add more elements to try and neutralize it, at least in the center. Each lens is ground imperfectly and the glass has flaws too. You keep adding them, usually 5 or 7 elements to the point where there are diminishing returns. It will never produce a better image than the bare lens.
Again, it depends on what you are comparing and I explained the
specific case where adding this adapter will lead to
sharper results with potentially
slightly less chromatic aberrations than without it. Will the flare become worse with the extra elements? Will the bokeh rendering change? Possible.
There are quite a few comparisons regarding the Metabones Speed Booster which you can look up if you want.
With the Canon adapter there are no side-by-side examples as of yet, but on the first look video, it looks to be even a bit better than the Metabones (maybe because it is even more optmised for the Canon sensor size)