If I remember correctly part of the "sensation" here was the 50mm 1.4 Art was optically wonderful but couldn't AF well enough on the 5DIII. Many reviewers praised it without mentioning the AF problems on Canon bodies, but I tried it and had unreliable AF. I sent mine back and occasionally wondered, a few years later, if it would have been any better on the 5DIV.It's hard to be definitive about "almost as good". There are differences; you'll need to decide for yourself how important they are...
By any normal standard the optical performance of the Sigma Art is extraordinarily good - it was considered something of a sensation within the industry when it was launched. Speaking personally, it is not important to me that marginally better might be available elsewhere. However it might be important to you; some people prefer to know that they are using "the best". Or there may be reasons other than sharpness to prefer the Canon, such as bokeh or ergonomics.
On the other hand, I had a Sigma 35mm 1.4 Art that did work fine on the 5DIII and even a little better on the 5DIV.
Sure would be great to hear from owners of any R body who use the Sigma 50mm 1.4 Art how well it AF's now.