One benefit for me was that a vps with 2vcors and 2-4gig of ram is about the same price as one of these app engine services. So I can run 3 or 4 dokku apps including Postgres, redis, memcache connected to them on my own vps and still have margin when inspiration strikes. I moved a production app from heroku to dokku and saved hundreds a month and still got tons and tons more compute.

In terms of money it might end up cheaper for the infra itself (especially if it runs in your garage)

In term of your personal time, well, you'll not grow a business on it without hiring people to maintain it or spend a lot of time doing devops