Maybe it’s possible with MkIV to check if the text before the \unit command was a number but of the number is part of the unit them put in the command, such things are always tricky with TeX and it’s better to force users to use proper input.
Within \type{phys-dim.lua} all the units and all the prefixes seem to have capitalised names; in fact, they should be all lowercase (even when they are named after some person). The exception is Celsius.
I suggested a list to Hans some time ago, so more than one person has this opinion...
I think this is a feature because Hans saves also a lowercase version of all keywords and you can use both as input.
I wonder whether \tex{unit} should only parse and format units, and have another macro \tex{quan} or \tex{quantity} to handle number+unit combinations (obviously using \tex{digit} and \tex{unit}).
I put my solution to this (expressing physical quantities) on the wiki a while ago….