Ranked in these QuestionsQuestion Ranking
Pro Customisable in every detail
Lilypond is easily the most powerful free/libre engraver, and arguably even more powerful than the commercial heavyweights Sibelius and Finale, mostly because it is set up as a Turing-complete programming language: it can be extended for any notation task whatsoever. For most common tasks this is of course not necessary (Lilypond has them built in), but whenever some feature doesn't exist yet, the user can just add it.
Pro Very very lightweight
Even more lightweight than MuseScore, just need the binary, some libraries (that come preinstalled with almost all systems), and a text editor (even ed will do).
Pro Best Typographical placement of musical elements, most readable engraving
Although there is a learning curve, the output of Lilypond is superior in every way. Very little after engraving adjustment is needed and the typography is both beautiful and very readable.
Pro Most stable software & files
Lilypond is quite stable as a software. As files are text files they're virtually incorruptible, the files could be edited with any text editor.
Con There's a big learning curve at first
Con Even for fans of text-based languages, Lilypond is not easy to use
Lilypond's syntax is influenced by both LaTeX and Lisp, and one might argue that it combines the disadvantages of both: it is both inconsistent and verbose. For simple tasks, ABCjs is more convenient to write, whereas for big projects a proper AST layout would be desirable.