Speaking of plain TeX:
Yes, they support UTF-8; that is, support it with a command-line switch which is a bit tricky to use. It's not like TeX can detect the encoding of your file and set things up automatically.
There's also the fact that LaTeX is often bundled in, and that is another "monstrosity" in itself. (I exclusively use plain TeX, so I have no need for that either.)
When I tried last time a few years ago it was not as complete as LaTeX is, atleast for me, who is much more proficient in LaTex.
You can also click somewhere in the PDF view and 'jump to source'. Used it a few times when polishing/debugging a LaTeX document, I generally don't use it for writing though. (I prefer to do all my writing in Vim).
That's an argument against using it, it seems to me. Having the words jump around as you type them is maddening.