I think it is fine to require from a caller that they are aware that a
byte (or byte array) must be passed to i2c functions. Given the (maybe)
two problems I pointed out above making it a bit harder to pass non-byte
data to these functions doesn't sound like a bad idea to me.

Obviously your mileage varies, but I personally like having an explicit
type in the API. I guess we have to agree to not agree and let Wolfram
decide if he likes your change or not.

I am on Uwe's side here. I like it being explicit and think the casts as
they are now are the smaller problem.