And why doesn't the union reference operator work in this context?

=COUNTIF((A1:F1,M1:Z1),10)

Yes, I know the workaround in this case: the sum of two COUNTIFs.
That's not the issue. The point is: the union reference operator
works with some functions -- e.g. IRR((A1,B1:B5,C1),7%) -- but not
with others. Why?