I spoke with Peer (the creator of Grisp) about this at Elixirconf earlier in the year, and I'm not an expert here so I hope I don't misrepresent his comments:

Grisp puts enough controls on the runtime that soft-realtime becomes hard-realtime for all intents and purposes, outside of problems that also cause errors in hard-realtime systems.

(Also, thanks Peer for being tremendously patient with a new embedded developer! That kind of friendly open chat is a huge draw to the Elixir community)