The code
\usemodule[units]
\starttext
$y = \underbar{20 \Mega\Bit\Per\Second}$.
\stoptext
produces space before y and it and y get underbared. MkII produces an expected result.
On the other hand, in
\starttext
$y = \underbar{g a w}$.
\stoptext
underbar doesn't appear (other letter combinations I tried seem to work). MkII can't compile this and ends with ! Incompatible list can't be unboxed.
--
Vedran Miletić