When pushing Products to Mailchimp code field from Sylius is used as id in Mailchimp.
Mailchimp id field has (probably) undocumented requirement - it can't be longer then 50 chars. The problem is that Mailchimp is not throwing any error - instead it trims given value to 50 chars. So when code field is long enought it may generate quirks and errors while pushing products to Mailchimp.
For example:
Sylius Product:
code: zestaw_dos_argentinos_playadito_amanda_despalada_2kg
Mailchimp:
id: zestaw_dos_argentinos_playadito_amanda_despalada_2
So if there will be product in sylius with code like:
code: zestaw_dos_argentinos_playadito_amanda_despalada_200g
then Mailchimp will trim it to the exact same id creating conflict.
If Sylius id field is better choice, please remember, that Mailchimp id field has to be string.
I didn't check if other Mailchimp e-commerce models has same behavior (orders/customers/carts/etc).
Cart is not integrated for now. Order is using number field as Mailchimp Id, Customer is using id field, so we are good there.