> Solo people working on projects aren't too bad so long as they communicate the work they're doing and document it well.

Strongly disagree, been there, hurt like hell. Solo people on projects mean that other people can't give meaningful reviews (because they don't know the project that well, and because they have their own work), leading to 1. developer not getting good feedback and improving, 2. higher risk of them going down some rabbit holes, 3. bus factor of one and stressful vacations, 4. less guarantee that the docs and the code are any good, 5. a danger that this solo developer will turn into one of those "knowledge hoarders".

Solo development is never OK (imnsho). Sometimes you can't avoid it, but it should be a calculated risk, and only a temporary one.