Let's be clear - scientific-grade code is a substandard of production-grade code.
But it is still a real standard.
Does scientific-grade code need to handle a large number of users running it at the same time? Probably not a genuine concern, since those users will run their own copies of the code on their own hardware, and it's not necessary or relevant for users to see the same networked results from the same instance of the program running on a central machine.
Does scientific-grade code need to publish telemetry? Eh, usually no. Set up alerting so that on-call engineers can be paged when (not if) it falls over? Nope.
Does scientific-grade code need to handle the authorization and authentication of users? Nope.
Does scientific-grade code need to be reproducible? Yes. Fundamentally yes. The reproducibility of results is core to the scientific method. Yes, that includes Monte Carlo code, when there is no such thing as truly random number generation on contemporary computers, only pseudorandom number generation, and what matters for cryptographic purposes is that the seed numbers for the pseudorandom generation are sufficiently hidden / unknown. For scientific purposes, the seed numbers should be published on purpose, so that a) the exact results you found, sufficiently random as they are for the purpose of your experiment, can still be independently verified by a peer reviewer, b) a peer reviewer can intentionally decide to pick a different seed value, which will lead to different results but should still lead to the same conclusion if your decision to reject / refuse to reject the null hypothesis was correct.
Does scientific-grade code need to handle a large number of users running it at the same time? Probably not a genuine concern, since those users will run their own copies of the code on their own hardware, and it's not necessary or relevant for users to see the same networked results from the same instance of the program running on a central machine.
Does scientific-grade code need to publish telemetry? Eh, usually no. Set up alerting so that on-call engineers can be paged when (not if) it falls over? Nope.
Does scientific-grade code need to handle the authorization and authentication of users? Nope.
Does scientific-grade code need to be reproducible? Yes. Fundamentally yes. The reproducibility of results is core to the scientific method. Yes, that includes Monte Carlo code, when there is no such thing as truly random number generation on contemporary computers, only pseudorandom number generation, and what matters for cryptographic purposes is that the seed numbers for the pseudorandom generation are sufficiently hidden / unknown. For scientific purposes, the seed numbers should be published on purpose, so that a) the exact results you found, sufficiently random as they are for the purpose of your experiment, can still be independently verified by a peer reviewer, b) a peer reviewer can intentionally decide to pick a different seed value, which will lead to different results but should still lead to the same conclusion if your decision to reject / refuse to reject the null hypothesis was correct.