I always find it very brave those who decide to use a newly created database in their projects.
Postgres jsonb works well enough for me - but this looks like a good alternative for Mongo people.
Based on this description I'd agree that FerretDB isn't a database itself. However the conversion between the MongoDB wire protocol to SQL queries could have bugs, data resiliency could be an issue if you need to guarantee writes, No guarantees of on-going support, etc.
New db's are always welcome but to use a brand new one in production would be very.... bold.