There's no other lens comparable to the Sigma 18-35mm f/1.8 Art currently on the market. That is, there is no other zoom lens in that focal length range with a maximum aperture larger than f/2.8 that is offered in the Canon EF mount. In fact, there is no other zoom lens in any focal length range with a maximum aperture larger than f/2.8 offered in the Canon EF mount.
The lens should be compatible. It will certainly mount on your camera. By all accounts it will communicate with your camera regarding the electronically controlled aperture. By all accounts it will AF with your camera. The question really is, "How well will it AF with your camera?"
The same exact lens will perform differently on different camera bodies. The variations will probably be greater on different models, but there will be some variation between different copies of the same model. Different lenses will perform differently on the same exact camera. Again, the variations will probably be greater on different lens models, but there will be some variation between different copies of the same lens model.
But here's the thing about phase detection autofocus (PDAF): no matter what camera body and what lens you have mounted on it PDAF is never 'perfect'. A well implemented contrast detection autofocus (CDAF) system will almost always be more accurate. So will careful manual focusing using magnified Live View. The reason we still use PDAF many times is because it is fast, it works with a TTL optical viewfinder, and usually it gets 'close enough' for most purposes.
Anytime you buy a 'third party' lens, you risk incompatibilities with newer camera models released after the lens was made. This is because third party lenses are reverse engineered and then tested with existing bodies available at the time they are developed. Companies such as Sigma and Tamron will often release an updated firmware for the lens(es) in question to resolve the issue.
They usually will modify the lens at no charge, at least until the lens has been discontinued for a while. In the past this meant the hassle (and cost) of shipping the lens back to the maker or an authorized service center to have the updated firmware installed in the lens.
One thing that has made this risk more acceptable is the development of USB docks that allow the end user to update the lens' firmware themselves without needed to send to the lens off to a factory service center. All of the Sigma Global Vision lenses, including the 18-35mm f/1.8, are compatible with the very affordable Sigma dock. The end user can download the updated firmware from the manufacturer and then use the USB dock to install it on their lens.
If there are actual compatibility problems between this lens and some of the newer Canon cameras it is very likely that Sigma will release a firmware update to rectify the issue.
What often happens on 'the internet', though, is that other issues are often (but far from always) at play when images turn out blurry. What one commenter may interpret as a 'compatibility' issue may, in fact, be one of these other issues. If one has been shooting with f/3.5-5.6 and f/4-5.6 zoom lenses and then one starts shooting with an f/1.8 lens, there will likely be a need for increasing one's knowledge and skill using the same camera's AF system because the margin for error is dramatically smaller.
The other thing that the USB dock enables is very detailed calibration of the lens for a specific camera body. As the resolution of camera sensors and the lenses used with them have improved, we are beginning to be able to see the effects of very small manufacturing variations from one camera to the next and one lens to the next in the pictures we take. Previously, it took an expensive test bench to detect such variations. The Sigma Optimization Pro application allows the end user to adjust the lens' AF calibration to ake these variations into account. This is discussed in detail in Bryan Carnathan's Sigma 18-35mm f/1.8 DC HSM Art Lens Review at The-Digital-Picture.
For further reading on AF systems, Roger Cicala's Autofocus Reality series is very insightful. Part 1, Part 2, Part 3A, Part 3B, and Part 4.
And: How Auto Focus (Often) Works
Also: Are zooms always sharper at one end than the other?
