ExtJS vs React

That ExtJs is expensive is not the problem, but what they did is started completely free, then switched the licensing model when they had profited from the community. Also, they suddenly switched from a single user license to a minimum of 5 users.

Ext-JS is not for free, it comes with a license.

The library is almost 500Kb in size hence significantly affects your page load time.

ExtJS is free for use in open source projects, but you have to use the unstable version which is riddled with bugs. Furthermore, to build a project (even if it's open source), you have to buy their proprietary tools.

For commercial projects, it costs $665 for a license.

You only get a limited premium questions that could be answered via the support and if the issue is too large then you might lose all your credits (it has a credit system for support) in one question.
In addition the replies in premium forum are sometimes less thought of than you would get in stackoverflow.

The community is very small and this is because the licencing is very expensive. Therefore not many people to test the framework and give feedback in order to fix.
They try to do everything in house which means that a small company (linkedin shows about 100 employees) tries to imitate something very large.
The scaling problems lead to lots of bugs and instability.

ExtJS does not take into account the asynchronous nature of todays web browsers where you can do heavy stuff on the background.

They had a GUI architect tool, but that tool never was up to date with the current framework version. Now they even seem to have abandoned the whole architect app. Also, to build a ExtJs app you have to use their tool (and then you have to pay, from that moment ExtJs is no longer GPL). But this tool also has lots of bugs.

ExtJs is a very opinionated framework. You have to do it the ExtJs way or it's the highway. So, a lot of time, things in your own opinion doesn't make sense or don't work in the way you expect it.

There is a large amount of source code that can be useful and indeed all the samples works flawlessly.
When you try to get into more complex scenarios you will find yourself in situations where things just do not work as expected. You will have to do manual work.

React's virtual DOM is fast, but it requires storing elements in the virtual and real DOM increasing memory usage for the page. This can be a real problem for single-page webapps designed to be left running in the background.

React gets a little verbose as applications get more complicated with more components. It's simply not as straightforward as simply writing HTML and JavaScript would be.

Requires learning a custom syntax, JSX, that has some gotchas and introduce complexity, a steeper learning curve, and incompatibility with other tools.

Though you can opt out from JSX and use vanilla JS instead. But that is not recommended since it adds a lot of unneeded complexity which JSX tries to avoid.

React does not do everything for the developer, it's merely a tool for building the UI of a web app. It does not have support for routing or models, at least not out of the box. While some missing features can be added through libraries, to start using React and use it in production, you still would need to have experience, or at least a good grasp on what the best libraries to use would be.

React's react.min.js is 145.5KB in size. It's much larger than some other libraries that offer roughly the same features and it's almost the same size as some MV* frameworks such as Angular or Ember that offer more features out of the box.

Although, it should be mentioned that sometimes having a smaller library may force developers to reinvent the wheel and write inefficient implementations on features that React already has. Ending up with a larger application that's harder to maintain and/or that has bad performance.

React has recently dropped support for Internet Explorer 8. While the library may still work on IE8, issues that affect only IE8 will not be prioritized and/or solved.