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.
It is pretty common in the embedded world in smaller companies. There just aren't enough developers to render enough support.
The problem with one-man-shows is the dependability. A lot of web developers go that route. But if they are unavailable because of private stuff or illness, the project they are working on is basically iced.
What would you say, in a team of 6 devs, if a pm just walked up and said "If we can get 6 different tasks done in a week versus 3, guess which one I'm gonna pick?"
Some people just work better on their own.
IMO, what's worse are "knowledge hoarders". Usually they've been in the company for a long time and they maintain their position by being as stingy as possible with their knowledge. You usually find these people in big orgs, and they can be quite toxic.
They also don't write good documentation.