The Haiku platform is a bit a second-class citizen in Godot right now, as it was contributed ~1 year ago but since then we haven't had a regular contributor working on maintaining it. Noone seems to be using Haiku in the core development team, so there's no strong focus on it.
Building the editor should work (but again, not sure to what extent it has been tested recently).
As for exporting, Haiku likes the instructions to do so (see e.g. the
export folder in platform/x11, which is missing in platform/haiku). It could likely be added without too much effort by a contributor using Haiku - as soon as we'd have one or two regular contributors available to maintain the platform, it could be added as a supported platform out of the box and we could maybe even build official export templates for it.