These sort of projects tend to be pretty large and also fairly binary: either it's complete, useful, and playable, or it's not (yet) complete and it's unplayable and useless. There's a bit of grey area, but a lot less than many other projects.

Kinda hard disagree there, I guess. I’m following both Dethrace which is source port of Carmageddon, and OpenGothic2 which is source port of Gothic 2. And both of those projects were massive fun to play before even reaching feature parity. You just need to get your project to the point where game can be completed and there is no crash every 5 minutes.