I can't think of lack of compatibility, unless someone somehow has an almost-a-date string they want to detect from a $null value.
Enabling comma as a delimiter would enable a very common date format
//var -s %a Mar 25, 2008 , %b $ctime(%a) , %c $asctime(%b)
//var -s %a March 25, 2008 , %b $ctime(%a) , %c $asctime(%b)
Looking at the page for https://en.wikipedia.org/wiki/Date_format_by_country
It would be crazy to try to support them all. I do see that the period is a common delimiter used by multiple countries. While mIRC is recognizing them in some formats, one I see used several places that's returning null is: (ddd|dddd) (d|dd). (mmm|mmmm) yyyy
//var -s %a Tuesday 25. Mar 2008 , %b $ctime(%a) , %c $asctime(%b)