If you have an apostrophe in the comment it produces an invalid command.
useradd -c 'John Smith's Account' ...
The above command now has an open quote.
Good catch! Since the user_account delegates the hard work down to the user resource (which ships with Chef), this is actually a Chef bug. I found a tracking ticket CHEF-3062 but it doesn't appear to have a pull request associated with it.
I may try to give the ticket some love, but in the meantime I'd like to close this one out since there isn't going to be a clever way to account for it in this cookbook.
Thank you again--I'm sure others will find this useful for reference (as will I)