I agree, Sikuli alone isn't do the trick. You have to combine many testing techniques. You have to use every tool available to test your software, mobile or desktop, and your website as good and hard as you can.
I recently learned if you develop software for embedded devices they use all available tricks to test their deviced, so they are fool proof. So why not combining sikuli with selenium or integrate sikuli into junit maven tests. I don't see a problem here.
I for myself have decided to give Sikuli a go and I wrote a litte BDD Testframework "JePySi" to test my websites. I created a couple of standard VMs with jenkins slaves and the run the tests triggered by a CI/CD job construct on a Jenkins master. And so far I'm very pleased with it.
Of course you have to overcome some obstacles. The images eg. Retina screenshots do not work at all with sikuli. You have to take screenshots with the Sikuli IDE, so they work. And Yes sometimes scripts fail. But most of the time, because there are OS differences, app differences or rendering differences. But that's the challenge I got mostly tackled with my framework.
E.g. allow different images on different OS enviroments for the same image. E.g. the green SSL header from bitbucket.org is rendered differently on mac and windows. So I give them a name like "bitbucket.org header" and in the background I've two images one with the mac rendering and one with the windows or linux image. And I wrote a complete Testsuite for jepysi.arjs.net within one day.
Yes sikuli is great for apps and websites and Yes selenium is also a very good tool for website testing and not so much for apps. In my opinion you should use what you think get the job done. I prever sikuli for now.
Btw. I open sourced "JePySi" on bitbucket.org, since I believe in open source. So if you interessted give it a spin.