This is much better for overall scriptability IMHO than AppleScript
Then StickyKeys etc are valiant attempts at a complex area with so many different user types
Pretty much anything used to drive a UI on OS X is going through the accessibility APIs to do so.
So for instance I will have a button on my switch control on-screen keyboard, and when it's pressed it sends a request To If This Than That, which communicates with my LIFX lightbulbs and flashes all of them in the house blue notifying my carer that I need assistance. That's just one of many. I love AppleScript.
To be clear I control my computer with my right index finger which gives me left mouse click, a small reflective doton my glasses which gives the cursor movement and voice dictation software which enables you to leave comments on Hacker News.
On Windows, Autohotkey is my goto for automation
That I control tech exists for every platform, but there is only OS X that you can take out of the box as a quadriplegic and with minimal help from a Designated Pair of Hands™ be up and running and independently messing around on HN.
Believe me, if I could buy a cheap laptop running Windows or Linux preferably and get the same user experience I get from OS X I would do it in a heartbeat. But in the past decade of trying, nothing comes close to OS X at the moment and even then OS X has only been this accessible for the past two or three additions.
All Microsoft did in this instance you're talking about is make sure that the drivers worked, and I trackers are essentially just mice as far as the computer is concerned, all of the clever stuff is done inside the eye tracker itself.
Just my two pence worth. But you know, It's 2 pence that I've earned through a decade of being a quadriplegic geek getting very annoyed at not being able to use my computer like everybody else.
I long for the day that I could use voice dictation software on Linux, any Linux distribution would be fine, then I could switch to some nice open source software and I would be a very happy pussycat. But none exists as far as I'm aware.
Edited to add: for clarity and excuse the odd grammatical mistake, Dragon doesn't make spelling mistakes but it does put the wrong word in the wrong place sometimes.