Nah.. I don't know about the others but Knockout was super simple to learn and get going with. React was way harder IMHO.
ReactJS of the time was pretty simple.
https://web.archive.org/web/20131105184444/http://facebook.g...
You could read every scrap of documentation in around an hour or so. There were only around a dozen-ish APIs to learn along with only a couple "unique" concepts like controlled inputs, refs, and JSX.
Meanwhile, there was never a need to step into React code unless you believed React itself was buggy (I only came across this one time and the bug had already been reported and was being worked on). Even better, React code was usually faster than Knockout before optimizing stuff and the ways to improve performance were generally pretty obvious.
It took MONTHS to become proficient at early-ish frameworks like GWT, Dojo, Marionette, Angular, Knockout, Ember, Enyo, etc.
Getting up and running on React in a couple hours and almost completely understanding the entire API inside a couple days was incredible. The fact that the code was faster and had fewer bugs too was almost too much to believe.