I haven't had a lot of experience with controlling Numbers by Voice, but the main problem seems to be that Numbers doesn't have a vocabulary for function names (I'm not even sure there's agreement on how some of the more esoteric function names should be pronounced :)
The two things that got me most productive was either speaking functions by name - e.g. "Type - S - U - M - open parentheses - B - close parentheses", or via 'show numbers' and navigating the function tree numerically. Unfortunately the numbers for the sidebar items vary based on the contents of the current sheet, so there's no memory recall to help.
I also couldn't find a reliable way to switch focus between the formula entry box on the sheet and the function list on the sidebar, although that might just need a little more familiarity.
It's probably worth adding feedback here: Feedback - Numbers - Apple and hopefully a future version will make this easier.