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