I have written my Bachelor thesis in org-mode and was incredibly happy with it. There is certainly a difference in requirements for a PhD Thesis, but I cannot think of any missing feature or hard to overcome problem right away.
I usually use perl/sed/awk + markdown for generating html from my own made-up mini-formats. I'd love to keep the sources in org-mode instead, but I wonder whether elisp is the right language for the type of text munging I want to do.
Modifying the HTML output (adding classes etc.) was fairly OK, I haven't tried anything crazy though.