How possible is it for an experienced developer who has created games in both unity and unreal by himself to create a game in xenko?
Yes, it’s possible.
You should read this if you’re Unity developer:
http://doc.xenko.com/latest/manual/xenko-for-unity-developers/index.html
There’s lots of DIY if you want to learn Xenko, as this forum is feels like “empty town” compared to Unity official forum, even unofficial forum.
There’s a time when you ask question, someone just say read to doc, or you must skim through Xenko source code because no one reply to your question, or because they think your question too stupid to answer, 
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If you’re an experienced developer with both Unity and Unreal, developing with Xenko should be a breeze for you.
As @F8R said, there is a lot of DIY and you will have to work a lot with Matrices and Quaternions if you want things to work properly (thing that does not happen a lot at all with UE4, probably a bit more with Unity).
Other than that, there is some Documentation, at least of the API, so you may have to dig into the sources sometimes to get a grasp at how something works. BUT. On the contrary of Unity, the Source code of the Engine is open-sourced, so you will find it easy to go to the deepest levels of the way Xenko works (up to the moment it calls native DLLs at least).
If you’ve ever took the habit of extending Unity’s interface with tools however, you will have to wait for a bit as this is not something possible to do just yet (I’m not talking about UE4 since extending its UI was just so much of a pain to do the last time I checked, a couple major revisions back).
Overally, I think that’s possible to do without it being extremely hard. As a developer, my only struggle really so far has been arts, because, well, I’m not an artist. 
Regarding my experience so far with the Forums, it’s not extremely lively, but I had more answers here than I had on UE4’s answerhub (because most of the time my questions are in the boundaries of the engine capabilities) and Xenko devs seem to be taking a look at it as well. 
Hope that helps a bit.
PS: async/wait FTW.
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