It's horrible for performance. A iso8601 timestamp takes at least double the amount of bytes (without a timezone), but more importantly you need to parse and validate the timestamp each time you use it. Filtering and sorting suffer a lot. Iso8601 without timezones can in theory be sorted trivially without parsing, but sorting strings is still a lot more expensive than sorting arrays of int64. And you also have to make sure to never write a different timezone. Why not just use an int?

You then have to parse the string into the native datetime type again in application code.

You also need to add a custom check constraint to prevent writing garbage to the column, which is easy to forget.