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 ß, ü or 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.
\usepackageIIRC. So switching from LaTeX to XeTeX would require changing existing documents. Don't forget that you can use unicode in LaTeX as well, at least to a certain extent. And when it comes to fonts, most people seem to be happy with what LaTeX provides -- this is the case for me at least. I do not want to worry about fonts and about how they go together. I take what is set up in my favoritedocumentclass, because the guy who wrote it, knows more about it than me. – Philipp Imhof Aug 31 '15 at 12:07\usepackage. – egreg Aug 31 '15 at 12:16microtypesupport :) – cgnieder Aug 31 '15 at 12:33pdfTeXfor its compilation times. – 1010011010 Aug 31 '15 at 22:45\pdf...commands. Speed, microtype, TikZ and custom font configurations mean I use pdfTeX for everything serious, though. But I only really need English, Welsh and odd words/phrases from Greek and other languages. I never use command macros for accented characters etc., though (except in maths) or for Greek words (except in maths). I input all those directly as unicode, which works fine for me in pdfTeX (except ŵ, ŷ¸ Ŵ and Ŷ are not supported out-of-the-box). LuaTeX is tempting, though.... – cfr Aug 31 '15 at 23:51externalize-command LuaLaTeX to plot measured values etc.. – jlk Sep 01 '15 at 00:30xdv, which is (usually) converted directly to PDF byxdvipdfmx. This is much more like the DVI route with pdfTeX than the PDF route. – Joseph Wright Sep 02 '15 at 12:10