That about sums it up. Coarray fortran is a «competitor» that seems a lot easier to use.

its a shared memory programming paradigm that can be implemented on distributed memory machines. UPC programs dont have strong any consistency guarantees. it does have a formal memory model so you know what semantics you can depend on.

its kind of a cultural successor to the SHMEM programming model which I think was developed in the 90s for the Cray t3e. at least until recently it was used quite by the NSA, but LBNL has been investing in it for general scientific use (multigrids has been a focus)

the previous environment was open source, so I'm not sure what moving to a gnu project changes. the old compiler was getting long it the tooth, so its probably just a refresh of the compiler base.