I'm a heavy LaTeX user, but LaTeX manages to be simultaneously amazing and completely insane.
Let's get started with some negatives (the positive are well known):
- Recompilations needed. Can be mitigated by latexmk, but still.
- Choice of latex, pdflatex, xelatex, lualatex ... and documents compiled with one may or may not compile with another
- packages that are incompatible with each other in odd ways
- when you encounter an error you have to enter "X". if you're using latexmk, you have to enter "X" multiple times
- math commands are not structure-based but presentation based, despite LaTeX's (otherwise well executed) posturing of separating content from styling. If you want to change the alignment of your equations or the size of the integrals sign or just slightly tweak the appearance of the | sign in {x^2 | x € R }, be prepared for a lot of rewriting (some of this can be alleviated by defining new commands, but I don't know how many people actually do that)
- Math LaTeX input is basically unparseable in the sense that it is not really possible to take some latex math code and convert it into some actual meaningful structure (this is a corollary of the above point). Given the ubiquity of LaTeX-style math even in a range of other contexts (e.g. KaTeX), this is a bit of an issue. It's possible to do a best effort implementation that sort-of works, but it's always gonna come up short
- Error messages: "Missing $", thanks
- Outdated defaults. Unicode should just work by default.
I think I could go on, but you get the point.