I will probably not use it, but might be interested, if it had an API. From my [admittedly, cursory] examination, it does not appear to have an API.
I assume it uses OSM data? But maybe not, as I don't see any attributions on the maps.
Personally, I am a "cartophile." I love maps, and really enjoy using them.
I have heard some folks throw shade on MapBox, but it appears to be a pretty good system, from what I have seen.
There are plenty [1] of sites & apps (some quite large) that don't correctly attribute OSM data according to the legal requirements [2].
[1]: https://wiki.openstreetmap.org/wiki/Lacking_proper_attributi... [2]: https://www.openstreetmap.org/copyright
There's some overlap in features but certainly Radar Chat doesn't have any of the drawing functionality.
The Postman collection thing is something I've been seeing, recently. Great idea.