If I recall, D2D doesn't provide much flexibility. Tools like Vellum or Atticus (allegedly--I haven't used it--provide for a broader range of formatting options.
It's true, though, that if one was content with the D2D formatting, that does seem like a simpler solution.
Vellum is only MAC; I have no experience with the app. I have tested Atticus a couple of times. The first time, it was 'primitive.' Months later, the second review was better, but not enough to pay and switch.
To Tim's point, I can format manuscripts using MS Word (Docx) perfectly for print. The tools for ePub are "good enough."
Another reason I am seeking an improved tool for ePub creation is that my style intentionally uses white space more than most fiction creators.
Too often, the ePub tools insert and remove blank lines. Again, I have the skills (XHTML and others) that I use to "correct" the output files. So... PIA.
One more thing?Version 3.3 of the ePub "standard" allows the insertion of WebP and Opus video formats. Foreshadowing: Assuming the security issues are resolved, it will soon be common for eBooks to contain embedded videos. Think of opening a book on an e-reader, flipping to chapter one, and watching/listening to the main character present the back story in a prologue format.
R.C.