Because I feel like the Source Editor is getting the short straw of the UCP updates in lieu of the Visual editor, I guess I should leave some feedback about things that need to be added and/or improved to it. Source Editor is what the majority of veteran editors use to edit and shouldn't be left behind.
Link Auto-completion[]
One of the best conveniences of the Legacy editor was the ability to type in a hyperlink or template and the system would attempt to autocomplete it based on existing pages/templates on the wiki. I really wish this can be implemented on UCP.
Category Field[]
On Legacy there is a special field on the right panel where you can automatically add categories to a page without having to type them on the main page's body. UCP has something similar, but for whatever reason its only enabled on the Visual Editor. I don't see a reason not to enable it on Source Editor as well.
Media Uploader[]
When you want to upload a picture on Legacy you click the corresponding button on the left and a dialogue will appear where you can either pick a previously uploaded image or upload a new one, then once uploaded another dialogue appears where you can choose to put it on a thumb and define dimensions. UCP adds in a bunch of extra unnecessary steps, to the point its easier to just use the traditional Special:Upload page then add the file on the page manually.
Gallery Builder[]
How in the hell is this supposed to work?
On Legacy the gallery builder works much like the Media Uploader and is just as intuitive, plus you have different buttons for different gallery types. As it is right now the UCP gallery builder is pretty much useless, it doesn't allow you to upload files through it and trying to search for previously uploaded flies through it doesn't work as well.
Video Uploader[]
Legacy as a simple but effective video uploader where you can insert the video link to add it to a page. Simply put there isn't a video uploader on UCP yet, you have to use another special page to upload it then add it on the page manually.
In closure[]
This is what I can remember for now, and I hope all of these points are improved while the UCP changes are implemented.