I'm not sure this can really be summed up like that.  Unfortunately DNN skinning still isn't as predictable as it should be.  I find that to fine tune a skin you usually need to work in both HTML and ASCX at some point in time.  Although I don't use HTML that much since I have no problem understanding the ASCX side, there are times when I just can't get what I want and I dummy it in a HTML environment.  Uploading that and grabbing the ASCX to do final tweaking usually does it for me.  I also have a habit of creating the basic structure of a new skin/site in HTML and uploading it, then working withy the ASCX.  The first HTML is nothing but the layout, not even images, but I can do this a whole lot quicker in HTML simply because I've been doing it so long.

So I guess my real point is:  Don't feel inadequate if you have to use HTML, or even if you find HTML easier for parts of the development.  As you get more experienced you'll find the ASCK is more familiar and a lot quicker.

Jeff