It's funny, I looked at the "Typesetting Mathematics -- User's Guide (Second Edition)" postscript document, and - at least with macOS' Preview - some big brackets are segmented (Neatroff brackets don't seem to do this, although I've seen it in other troff generated documents), and they even say this:
> Warning — square roots of tall quantities look lousy, because a root-sign big enough to cover the quantity is too dark and heavy
The solution is naturally to rewrite big roots as powers.
pic does seem close to Tikz, although I had to look in the GNU pic doco to figure out how to do colors. Even then, transparency didn't seem to be supported?
Heirloom actually looks the most useful/mature. At least the output looks pretty/someone cared enough to make the example files pretty, there's actual documentation. Limitations are still there (having to convert bitmaps to EPS?). I will say I'm at least slightly impressed by `gpresent`, which is like beamer (so for making presentations), and built-in hyphenation support.
I still don't get Neatroff. It's compatible with/implements a lot that Heirloom does, but then the font support is worse again? It's an impressive project though, the source is very readable, and RTL/LTR support. Less impressive is the lack of a license - I think it's ISC, based on a single comment, but who knows?
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A repository and a makefile are distinctly different than an installer. Random macro packages that may or may not be on GitHub are different than `tlmgr`. Piping stuff around and having to convert images is different than just one command. GUI editors. Example documents (like https://texample.net/). That is what I mean by ecosystem.
XeTeX outputs PDFs by default (granted, via xdvipdfmx), and can also include bitmaps directly (again, granted it needs graphicx or something). All TeX stuff isn't without it's warts, and seems overly complex (pdfTeX/XeTeX/XeLaTex/LuaTeX/ConTeXt, etc). But in practice, it kinda somehow just works (until it doesn't).
[0] https://github.com/rhaberkorn/gpresent