A major shortcoming with FormattableString is that the C# compiler is hard-coded to always prefer implicitly converting an interpolated string to a string, which makes it impossible to write extension-methods for FormattsbleString objects.

…and they also always default to formatting with CurrentCulture instead of InvarintCulture. Apparently this was by-design as interpolated strings were never originally intended for generating machine-readable strings.

Finally, there’s no way to perform common “mini-templating” with a FormattableString, such as repeating regions, show/hide regions within a string, or little things like inflection (e.g. rendering “{0:N} items” or “{0:N0} item” when arg0 is 1 or not.

I’m happy to see Java (finally) gain a similar feature, but it, like C#,s, seems… limited in its abilities.