The main difference is that the newer lenses compensate for more aspects. Chromatic aberration and corner sharpness being the most obvious ones, but potentially also vignetting (there has also been a trend for new lenses to ignore vignetting with the "fix it in post"-mentality).
I don't have any experience with the Leica lens, but I've got the Canon EF-50mm 1.2 (and 1.4) and both of them are very soft in the corners wide open and they have plenty of chromatic aberrations, which is a consequence of the compact double Gauss optical formula. The Sigma Art 50mm and the RF 50mm 1.2 has a completely different level of corner sharpness and CA.
The main difference is that the newer lenses compensate for more aspects. Chromatic aberration and corner sharpness being the most obvious ones, but potentially also vignetting (there has also been a trend for new lenses to ignore vignetting with the "fix it in post"-mentality).
I don't have any experience with the Leica lens, but I've got the Canon EF-50mm 1.2 (and 1.4) and both of them are very soft in the corners wide open and they have plenty of chromatic aberrations, which is a consequence of the compact double Gauss optical formula. The Sigma Art 50mm and the RF 50mm 1.2 has a completely different level of corner sharpness and CA.