Quick thoughts based on 1 hour of dicking around. Note that I have not even taken a look at how creatures, scripting, quests, etc. work at all, just the basics of level editing.
To me it is like a fusion of Unreal Engine and CryEngine. Unreal similarity comes from the asset browser as well as the reliance on having to define areas for a lot of things, for example navigation and environment effects. Materials editor is also kinda similar in concept. CryEngine similarity comes in the UI, like how you switch between various "tools" using the right-hand pane, and a few features like being able to paint colors on terrain instead of materials.
I don't know if I like certain aspects of the workflow. I think the content browser in Unreal is useful but it takes up too much space and can get confusing and disorganized; the asset browser in RedKit is very similar and shares its weaknesses and strengths. I really do not like having to click back and forth between the browser and the main viewport just to add different instances of objects, because it means you need 2-3 more extra clicks than if the asset browser was implemented as a sidebar or list view instead. The asset browser seems to depend on having a multi-monitor setup. Don't have one, too bad for you.
I am also the kind of person who likes to constantly go between editing terrain, placing objects, and so on, so having to open and close the various tools can get a little tedious for the way I approach things. At a big studio, where presumably you would have teams of individuals dedicated to making specific aspects of each level, this would be less of a problem because you would probably only be doing one or two types of tasks at a time.
Oh, and lack of easy initial setup to get started (i.e. it doesn't give you basic lighting and terrain, you have to create it yourself) and other little hitches and bugs here and there make it awkward and a bit inaccessible when judged as a mod tool. However I do think CD Projekt cleaned up the UI, as it's missing much of the "hundreds of obscure buttons everywhere" clusterfuck you tend to see on more pro tools, and is stripped down to relative basics. It's kind of weird in that the usability and workflow feels a lot like professional SDKs but it's still not quite as feature-rich as those, and lacks a good deal of the intimidation factor at first.