There is a department at my uni, where people learn Serbian. It's not without a reason. It's not just mlijeko/mleko difference. There are some kajkavian dialects which also use ekavian versions, but kajkavian dialects were never Serbian. Slovenian is also ekavian and has never been Serbian.
One Serbian woman who moved here (is now working as the lecturer for Arabic at my university) had to learn Croatian, since yes, we can understand each other, but we're not talking the same language. American/British English comparison is a bit too simplified.
One Serbian woman who moved here (is now working as the lecturer for Arabic at my university) had to learn Croatian, since yes, we can understand each other, but we're not talking the same language. American/British English comparison is a bit too simplified.