Yes! Forks are seamlessly supported, this feature is quite experimental right now but definitely working!
DangerJS is a library, codechecks is a platform. This means many things but most importantly DangerJS lacks "memory", you can't compare things that happened on the other commit. This is solved by Storage API in codechecks where you upload any artifacts so you can easy access them later. Using codechecks you can do "simple" dangerjs style checks like if file package.json was modified in this PR fail the build but you can do complicated stuff like: calculate visual difference betweeen screenshots from base and head branches.
Another issue is user onboarding process, we provide already existing GitHub app which makes onboarding process easy and familiar to our users. You install GitHub app on your repos, you copy secret value generated for you and put it into your CI config and you're ready to go. With Danger you need to create manually Github bot, generate access keys etc.
Other differences include: we use github checks API that it's way more powerful than putting comments in your code. We embrace plugin infrastructure where particular checks (like ex. visual regression) are reusable between projects and distributed as NPM packages.
All of these tools do just one particular thing (track build size, code coverage of visual regressions) so you need to maintain all of them. They require multiple secrets, repeated installations and every tool is configured differently. Sometimes config is not even part of your repository but users are forced to click through some UIs. Also, most of them are closed source and you can't easily fix things when they are broken.
On the other hand, with codechecks you keep whole configuration in one readable, declarative file and all you need to do is install add one secret. Furthermore, everything is open source and you can easily tweak particular checks. What's even more inspiring is that you can explore new ideas that were never done before like for example type coverage tracking in TypeScript projects.
Our whole platform is open source. You can use it and run on your own servers and never pay us any cent for this. But to create sustainable development model (ship new features, write new official checks, maintain already created ones) we provide SAAS product available at codechecks.io. codechecks.io is 100% free for any open source projects. We charge only for private repositories. We also provide consultancy services and premium support for self hosted instances for additional fees.