Greetings Shevek,
There's no need to be quite so cautious I think. Use convert-ly and see how you go. It's easy enough to visually scan a hundred pages output to check for errors - I do it all the time with my current piece of double that length.
I have written quite a few times on this list about this very topic. The 2.19 development line is very stable and highly to be recommended. not only is crashing behaviour very rare, but the scads of new features make it worthwhile if for nothing else.
I engrave scores by a colleague who is a major exponent of the New Complexity School, and this work pushes lilypond to various limits in all sorts of ways, and yet it holds up magnificently and produces very fine output indeed, quite astonishingly beautiful in fact. I am currently engraving a major piano sonata which is over 200 pages in length and well over a hundred thousand lines of lilypond source code, and never a hiccup. In the last few years I have only encountered in my work two small crashing bugs and these were very rapidly addressed by the fantastic development team.
As to version numbers, you will know that open source development projects are generally very humble with revision numbers, and are loth to bump them up without very good reason. 2.20 will eventually arrive, but there is no need to deprive yourself of the advances in the 2.19 series. It is a tribute to the maturity and skill of the developers in the lilypond community that the development releases are so stable and well produced. I cannot praise them highly enough.
In explicit answer to your question, there is no major problem lurking in 2.19 that you need to worry about.
On a practical note, I have the subjective impression the members of the list are more willing to assist in solving user issues and queries with the 2.19 series than 2.18 since it is more current - but I am prepared to be wrong about that!
Andrew