GRiSP serms to typically hit inbetween Nerves and AtomVM. Grisp have explored some really constrained hardware compared to the typical Nerves device.

AtomVM is to my knowledge a lot of steps further down since they run a much lighter VM than the BEAM. It is also much less proven though.

Nerves, Grisp and AtomVM all do some of the same things. They make some different trade-offs. Heck, Kry10 (proprietary but targets critical infra) even sort of approaches the same thing. Running Erlang/Elixir for resilient embedded systems.

I am on the Nerves team so you can guess my preference. But all are cool. And we all talk to each other. AtomVM is the most obvious complement to Nerves since they target different device classes.