I'd like to know why XeTeX hasn't (yet?) replaced other engines that lack features we take as granted in modern computing, like support for Unicode and modern font formats.
Is there any important downside in using XeTeX? Why would I want to use, say, pdfTeX instead?
EDIT: I wasn't asking for an overview on all the engines out there, but rather for precise reasons for why I wouldn't want to use XeTeX, since from my European perspective Unicode support is kind of a big deal. I work on a daily basis with documents in German, Italian and Czech, languages where non-ASCII characters are pretty common. I gave up on LaTeX for years before finding out that with XeTeX I didn't have to painfully remember to hardcode every
š I needed to use (which were a lot).
Also, as a type-lover I like to experiment with typefaces in my documents, so easy support for OTF and TTF fonts is a very nice thing to have in my opinion, a feeling I expected others to share.
Not having such features is a deal-breaker for me, and was genuinely looking for reasons on why would someone choose to use an engine other than XeTeX.