I think RPC in general is a ‘good idea’ insofar that it is another possible tool amongst multiple possible solutions. It definitely is not a silver bullet. Many of the things that people would use RPC for in other contexts might be solved in Elixir by other means by using its process-oriented architecture.
RPC’s main weakness is that it is invisible for the caller that a lot of work that depends on external, unpredictable, conditions is happening in the background, which means that you need to take care to keep your system remain fault-tolerant.
gRPC specifically seems interesting, but if you for instance already are sharing work in other response/request formats using e.g. RabbitMQ, you could leverage that instead of adding yet another moving piece of complexity to your application stack.