I know, you're telling me photoshop has the ability to create seamless gradients yet Illustrator can't. That's an unacceptable. It'd be great if they could at least make the vector colors pixel sized.

Many things can change, yes...
Again, it’d be hard to compare anything without the actual file on hands.
You can either share the part of it here in comments, or send privately to sharewithai@adobe.com, so the team can investigate.
In this case, put the link to this report in the text of the email, http://illustrator.uservoice.com/forums/601447/suggestions/49136090

There was banding, but not to this extent. It is disturbing even in a smaller view of the image, it is clearly visible even without magnification, in the past it was only visible when zoomed in. I upload graphics to Microstock sites, so I paid attention to this. I use the same operating system, the monitor was changed to the right, but based on the screenshot, the same thing is visible on other devices. Windows updates and video card updates, which are constantly coming out.

Tímea, to me this 'flaky' look seems absolutely normal for an art layered with gradients with blending modes applied. Illustrator supports only 8bit to blend these, and some banding is inevitable. But you say CS6 was able to pull it off, and it’s strange... can you share a fragment of your file, large enough to make a comparison? Do you compare it using the same machine, the same display, the same OS, or you claim it just based on your memories of it being smoother?

In an old Illustrator CS6 vector, the gradients became banded and fragmented when I opened them in the newer Illustrator. I layered the gradients with the screen blending mode. I used to have no problems with this vector file, but the problem appeared in my new Illustrator (29.1). What the problem could be?

Andrea, it looks like the gradient has too little difference between colors in stops, therefore banding happens... and Ai just can’t make the transition smooth because there is literally not enough colors to fill the distance.
Can you please share the test file with this gradient in it, just to verify it? Nothing else, just the object filled with the gradient.

Hi me and my team are having some problems with the gradients in illustrator.
When we try to make a simple gradient it seems like it was mad in steps and not clean and smooth as it used to be. Anyone is having the same problem or now how to fix it? P.S. I leave you a screenshot so u can understand the source of the problem.

J'ai le même souci et c'est vraiment pénible. Et je trouve que c'est encore pire avec la version 26.0.1.

Gradients have always been bad in Illustrator. If I need accurate gradients, I sometimes redo them in a rasterized file in Photoshop, which gives much better results,. of course this is not exactly a solution for most cases.

It would be great to see more a higher quality gradient in future versions, because it's really a strange weakness for such a developed piece of software.

YES! Exactly! I have noticed an increased gradient banding on the latest update. :( It has not been this noticeable before. On all color modes as well. Has some setting changed with the update, that I can reset or change?
Please let us know when this is addressed.
For our large format graphics company, this can cost us a lot of time. (having to correct the banded gradients in Photoshop before output)
Illustrator is great! (let's keep it it that way)

Mac OS High Sierra, Version 10.13.1
Adobe Illustrator Version: 23.0

For this linear gradient, my middle value is (0,0,0,90) and my two end values are (0,0,0,95)
The rendering shows up very blocky and striped.
All other color options appear to be blending fine. 
But most combinations of the really dark blacks and greys show up blocky like this.

There is banding from black and white when I do a gradient and it does not matter what size the object is I am doing the gradient onto.

A nice smooth gradient is important without banding. I have seen this done on many items, not sure what they used, but the gradients were so smooth and flowing.

I then tested it with white that went to 50% grey and still banding.

Why is the quality of the gradient tool so terrible in illustrator?

Also, why is it so complicated to get the color you want from the library, or any color into the gradient tool? Can you make the gradient tool work like it does in photoshop? That is a nice user interface. Course that one also has quality issues.

I know exactly what Raster Effects settings do, thanks, lowering it down is not the way — I still have to remember boost it up, when exporting final images. If AI could have a display quality slider, as Indesign has, that would help a bit though.