You are free to like the lens. But to me the pictures it takes look lifeless. And I tried to show what I meant with that using comparison pictures.
Unfortunately your pictures are not comparing apples to apples (explained above) therefore any findings are false.
It doesn't matter.
Well it perhaps does not matter to you but it matters a lot for the relevancy of such a comparison and possible results. In other words it is called biasing.
I have thousands of images I could show. It wouldn't make a difference to you. You like the lens. Honestly, I like it as well. But for me it's more of a special purpose lens. It's very good when you want that flat look but with lots of detail in the images. Like photographing textbooks. Or open landscapes in B&W where you want to create a surreal look.
Sure, if you use to stop it down to f/8... :-D .
No, my findings are not false. They are simply my findings and opinions. You don't have to agree with them if you don't like to.
Here it is not much about the subjectivity. It always rings the bells and throws off any trustworthiness if there is one individual claiming very strongly the absolute opposite from the rest of relevant world represented by very experienced, unbiased and well established reviewers around the world.
I much prefer my RF 50mm f1.8 STM to my 40mm Sigma.
Nothing against a subjective preferrence. I had that lens too but I sold it since it had no use from I bought 40/1.4 which is way better in every single respect.
I'm glad that you think so. Personally I think it has some pros over the Canon. But there aren't enough of them to justify the enormous size (and price) difference. The Canon is much simpler to bring along.
Yes, this is very relevant if size and weight are more important than IQ. I do not care about size and weight much since I somehow do accept that there is no free lunch: the brighter aperture, vastly superior IQ, AF, buld quality, weather resistance etc. logicaly have their price in terms of size and weight.
I actually did like the form factor of 50/1.8 but the struggle from its IQ was much stronger.
I do much prefer a 40mm focal length over 50mm, however.
Stopped down it is equally sharp (aside from in the extreme edges).
People usually do not buy f/1.4 lenses to primarily shoot them stopped down. Otherwise it makes no sense to pay for that expensive wide aperture.
And there is not a single aspect of 50/1.8 in which it would be better at f/1.8 than the 40/1.4 is at f/1.4. Sharpness central and peripheral, fringing, contrast, bokeh - everything WAY better with Sigma. Not mentioning the AF speed, build quality and weather resistance.
AF speed is excellent on this lens, I agree with that. Also build quality. It feels like a proper premium product. And it is a little bit sharper, but it is not perceivable to me unless I pixel-peep at at least 200%. At 100% zoom, I cannot tell a difference in the amount of detail that is captured.
This indicates some kind of a serious problem in your setup or attitude. 50/1.8 wide opened is soft, with significantly reduced contrast, somehow misty looking. This sample from the TDP review shows it clearly:
So on one side you "do not see" this which is very well known and very obvious. And on the other side you "see" lack of contrast, saturation and defintion on 40/1.4 which no one other sees. Hmm.
Still at f/2.8 the 50mm is not catching up in terms of contrast and sharpnes with 40/1.4 @1.4.
Add serious coma and astigmatism, its 7 blades creating hard-edged bokeh balls, overally busy bokeh...
Stopping down the Sigma to f/1.8 to match apertures the difference is even bigger and leaves 50/1.8 trailing far behind.
True, the Canon isn't that good at f1.8. Needs to stop down to 2.8 before they even out.
40/1.4 @2.8 offers much better resolution and contrast, especially in the wider center of the image. Also bokeh and the transition from focus plane to defocused is uncomparably better, smoother, more natural with the Sigma.
So if you like to shoot wide open, the Sigma might be better.
Yes, that usually is the purpose of wide aperture lenses to use them at wide apertures.
But then I would rather have the 35mm Sigma, which had much more pop to it (very similar to the Canon, but sharp from f1.4).
The 35/1.4 Art is also quite soft wide opened, although not as much as the RF50/1.8. Mixing softness with the pop ain't a good idea, I'd guess.
It has much more contrast and color saturation, and the images just looks better (in my opnion).
Could you share some RAWs shot with both lenses fully opened (same scene) which would confirm this? The overall worldwide experience is strictly opposite.
Why only fully open though?
Well that is what these lenses are made for

. There is no relevancy and logic to pay the premium for f/1.4 lens and then smother it at f/4. Then you get the flatness, lack of separation, much less pop...
But ok, if your level is asking what f/1.4 lenses are for when we after all have the IBIS, then the situation starts to be very clear... :-D .
To be honest, I very seldomly shoot wide open. I tend to shoot at f8 about 90% of the time. F4 if it is dark outside (IBIS in the R6 allows for second long handheld exposures anyway so there often isn't a need for more than f4).
OK, that is your prefference and that is fine. But then there is no point for you to spend money on premium wide aperture lenses because at f/8 almost every up to date lens deliver great sharpness and contrast.
People who look for bright lenses usually seek for the same assets when wide opened. There comes that huge difference between state of the art lenses as 40/1.4 and low end lenses like 50/1.8.
It is a shame that this is the case because I liked the older, smaller and cheaper 35mm a lot. Perhaps this problem is mainly there in the 28, 40 and 105mm lenses
As you can see from my samples, there are no such a problems you are writing about.
It's impossible to say with your pictures because there are no comparison pictures from another lens side by side.
Their purpose is not to be compared but to show you how loud these photos scream "we have no contrast, flat look, washed out colors, no separation" and all the other BS you made up.
These lenses seem to have been created solely to perform well on MTF charts, so that youtubers and other reviewers who shoot bricks walls and pixel-peep at 300% will praise them. But the trade-off for this is not only weight and bulk but a lack of soul and a sense of artificialness in the images they render.
I don't see any of it in my pictures. And never noticed anything like this from anyone who had an experience with these lenses. Seems you are the only one. Worth to think about.
Definitely not the only one. There are a lot of people who feel this way about Sigma.
Please share with us 2-3 well established respected reviewers who claim the 40/1.4 is flat, with poor contrast, lacking color saturation, lifeless and all the other BS fairytails.
Just curious

.
Just two another flat, lifeless no contrast photos whre 50/1.8 would do the same... :-D .
Hard to say from these examples. There is nothing to compare them to
That is not an intention, as explained above.
and they have probably had contrast etc added to them in post.
Of course, since I shoot RAW only the contrast had to be set on them somehow. The cat is -10 on contrast and the house is +5 on contrast in ACR. Different light and scenes require different processing. That might be quite clear for many...
The second picture looks like it has a bit of 3D to it, I will admit. But it may just be the way the scene is captured.
Well sure it is the way the scene is captured - the 40/1.4 itself captures very nice and palpable 3D pop.
My guess is if you had shot the same using an old EF 35mm f1.4L, it would've looked even more 3D-like.
Rather wishfull thinking than guessing. BTW the old (original) EF 35/1.4 lacked sharpenss and contrast wide opened quite clearly compared to the 40/1.4.
Again, would need to see comparison pictures to judge fairly.
In my opinion however (and you can see the comparison pictures posted previously),
Explained above why your comparisons are not relevant.
is that these are flat rendering lenses. If you like that, then get one. But to me it looks like a zoom render, not the rendering you'd normally want from a prime lens.
This all is just some kind of a strange way how to try to be interesting with with that selective sight and a huge level of denial.