But that CI is very easy to setup.

Take a given start point; feed a set of inputs; compute a checksum on the resulting state. Check that the checksum matches on different platforms.

And if your language is good you can parameterize your logic on a particular numeric type, so that a particular fixed point implementation is consistently used

Unless the GPU output is feeding back into the game loop simulation, there’s no nightmare.

I’ve worked on games with intentional determinism, and it’s a very valuable tool. A playtester can hand a dev a recording of the game inputs (including clock) for whatever bug they encountered, and the dev can fast forward to the exact point and repro it 100%. The amount of time saved chasing down difficult bugs is enormous.