I'm working on a 7 year old application now with 60+ past contributors, 3-4 teams working in the code base, 3,500 elixir files, and some semi-complex dependencies and I can honestly say that most of it is pretty easy to work on. We have one section that is full of some really thorny code, but that's because it relies heavily on code generation and that's rough in most languages. Onboarding usually only takes about a week, less sometimes.