I happen to have both this new 45/1.2 STM and the old EF 50/1.2L. I haven't run a full battery of tests, but what I can say from my limited comparisons is that the RF 45 is the sharper of the two, especially at wider apertures. Just a sharper lens.
On the other hand, the old 50/1.2L produces a creamier, smoother bokeh regardless of the harshness of the background. It is essentially a portrait lens. So 2 different animals with a similar purpose, perhaps, but one is considerably less expensive than the other is (or rather, was).
If I wanted ultimate sharpness, I'd get either the RF 50/1.4 or RF 50/1.2L lens - both appear to be spectacularly sharp and very well designed. But for my purposes, what I have now (along with the inexpensive 50/1.8 RF) will do the trick. I lean in favor of the RF 45 because of its size and weight, but I can't really say that I don't like what the EF 50/1.2L does for me either.
On focus shift, I hadn't paid a lot of attention to this issue, but I will be. Since I have the older R5, I don't have the ability to use "Display simulation" or whatever setting it is that people use to force the lens to autofocus with a smaller aperture than f/1.2. Whether this is a problem or not will be the subject of future testing and comparison to manual focusing.
