While most of it works great, I found that internally it sometimes uses the \note markup command, that changed between versions: in 2.20 it requires an argument of type string, while from 2.21 onwards it requires an argument of type duration. This causes an error and makes it so that files that use
arranger.ly do not compile on Lilypond 2.21 onwards.
I'm sure it'd be easy to patch. Is there an easy way to transform strings into durations?