I've tried the unit interface today, and I've noticed two issues with the
following example:

\setupunits[alternative=text]
\starttext
$\unit{-4.6e-3 km s^-1}$
\stoptext

1. Minus superscripts for UNITS (so 's^-1' here—not 'e-3', which is fine)
are converted to hyphens with the 'text' unit alternative (with the default
'mathematics' unit alternative everything is fine):

2. The times symbol between the number and the 10^-3 (e-3) is incorrectly
spaced (as an ordinary math atom instead of a binary math atom) so doesn't
look good.

Maybe Hans could change

\def\digitstimessymbol{\times}

to

\def\digitstimessymbol{\ifmmode\times\else${}\times{}$\fi}

in phys-dim.mkiv?