Yes, ISO-8601 is objectively better than YYYY/DD/MM or MM/DD/YYYY or other strange formats. But, because it isn't the most widely used format, it can be less efficient when there's no danger of MM/DD vs DD/MM confusion.

If person A is talking to person B and they're from the same culture, some culture-specific version like DD/MM/YY might be easier for them because it's what they're both used to, even if it is objectively inferior. But, if person A is talking to person C who is from a different place/culture things get more challenging because if the day of the month is 12 or under, it's hard to distinguish between DD/MM or MM/DD. Even if person C knows that person A comes from a place that does DD/MM, they don't know if person C used the native DD/MM format they're used to, or translated it to them to MM/DD because person A was trying to make things easy for person C.

One of the advantages of the hyphens in ISO-8601, aside from them working with files, is that hyphens in dates are less common than slashes, so if someone sees 1212-11-12 they're more likely to understand that it's an ISO-formatted date. If they see 1212/11/12 it might be a MM/DD date or a DD/MM date.