I am aware of the guide on their github
https://github.com/organicmaps/organicmaps/blob/master/docs/EXPERIMENTAL_PUBLIC_TRANSPORT_SUPPORT.md
But that is not something I would want to do for myself, much less so the people I usually recommend OM to :/
Is it just the lack of UI/UX or is there actually a quality issue regarding the GTFS/OSM source data?
yeah here is hoping that it comes along and becomes the default