React vs Sencha Ext JS

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.

To do any meaningful development, you are stuck with CMD. There is a gulp task that will handle the JS concatenation, but there is nothing outside of CMD that can handle theming in their ecosystem.

In addition, CMD is based on Java, and is very heavy to run (600MB+ on Windows 10 to watch for changes in the application and recompile).

Their latest release of CMD changed some configuration locations, but the documentation was not updated to reflect this. There is no reference guide on the json configuration files, other than the (unfortunate use of) comments in the generated json files.

On the plus side, you do not have to install ruby alongside CMD for more recent versions of ExtJS. However, their Fashion processor seems to only be available through CMD.

Compounded by the fact that there are now two "toolkits" in the same "version" of ExtJS, with certain components not existing in one vs the other.

You get all the IDE plugins, even if you only need one. They should offer sell them individually, or continue to bundle them with a dev license pack.

Any third party library you wish to include has to be wrapped in some sort of component adapter. You have to do a lot of tweaking to get the build process right if you want the 3rd party lib to be bundled into your application in the right order.

The framework is a commercial package, and the recent decision to start with a minimum of 5 users may rule out smaller developer teams or startups. Recently, they have started a program that allow essentially what are contractors to purchase single licenses, but not individual, independent developers.