The one thing I have learned about Canon over the years is that they accurately describe their cameras position in the line up.

So I believe the following that Canon has said.
If Canon says that the R3 is positioned lower than the 1Dx III then I it will be.
If they say it is less durable, it will be less durable.

By the specs we know that when shooting with a mechanical shutter the R3 will be slower.

So far we just have early reviews and discussions by those given pre release models. Usually those kind of reviews are lacking, after the body is out for 6 months or longer we will start hearing the real story.

If I were considering buying the best available body I would want to know how does the AF compare to the 1Dx III. I am guessing the 1Dx III will still be superior?
What is the expected battery life during shooting?
Does the additional demand on the battery that you have because of the electronic viewfinder take away from other performance criteria, for instance the speed that the 1D bodies are known for when driving the IS and AF motors of a big white?

When you are wanting the latest and greatest new item it is easy to overlook the minute indicators that the manufacture gives you.
When pre-ordering this body you would be buying what Canon considers their best RF mount body. You would not be buying what Canon considers its best body.