The Classic Editor can normally add and run any app from the Wix App Market full-time, while in EditorX they often switch their availability (often the ones developed by Wix) with long periods of unavailability, due to adjustments or bug-fixes. The periods are very variable and we rarely get any warning from the Team at all.
Although most apps are fully responsive (while available of course), or at least horizontally responsive, the Apps’ Settings DO NOT CASCADE DOWN through breakpoints; they may have a mobile layout (like Wix Music) that automatically activates under the 750px width, or otherwise, be the same across all breakpoints within their widget(s).
EditorX does not support Wix Multilingual nor a customizable 404-Error page.
Some Corvid functions from the API won’t work the same (or at all) in EditorX.
The mobile tools –as mentioned above by @pratham (logo screen and mobile browser custom color)– are not supported; although there is certainly a Mobile breakpoint for the sites you create, and Hamburger menus are available and stronger –in my opinion– than in the Classic Editor.
The Advanced Button Maker –which allows to combine text & icons, gradients (the only ones in Wix) and set individual contours & corners– is currently unavailable in EditorX. Image buttons and Hover boxes are also unavailable right now; however these are actually planned to come back in a near future within Interactions (see ’ Coming Soon '). For now, Text buttons are the only ones supported.
Slideshows are currently unavailable but on their way (see ’ Coming Soon ') too. Though you can get similar effects through the Wix Pro Gallery –if the idea’s not too complex– with its obvious limitations; or the same effects through coding.
All item animations are available and customizable in both editors, but it’s not the same case regarding the page-transitions : EditorX switches between pages within a site with a fade animation by default (which fades to white, independently of the page background colors selected) with no option to modify it or turn it off.
Normal image galleries –which offered some alternative layouts (like carousels or hexagons)– are also not supported: only the Wix Pro Gallery is supported. Additionally, the Wix Video App can only be shown as an horizontal strip: all other layouts are also not available yet but may be added in a future.
I also have to say that Wix allows images and video as page backgrounds with different effects each (Parallax / Reveal) and adjust their position and transparency; you can even set a graphic overlay on the video background. EditorX only allows plane solid colors as page backgrounds, and all of the above as section-backgrounds only, which has its obvious limitations.
These are the features (most of) unavailable as for today, January 4th of 2021, but this reality might (and is likely to) change as time passes and EditorX evolves.
There are many differences –beside EditorX’s obvious Responsivity – between the Wix Editor and EditorX; however, let us be clear that EditorX is still in BETA stage while the classic editor has at least 5ive years of constant development.
If any of the above listed –or any other that I may have forgotten– is really important for you, you can always submit a request through the Feedback Widget (EditorX > Help > Request a Feature) or post it in the Wish list of this same Forum.