U.S Startups hiring non-U.S citizens?

Hi there,

I was wondering how common is it for U.S startups to hire immigrants from Canada, Europe or anywhere else? For example, a Canadian citizen can come to the U.S to work on an Employment Visa (TN For example) and I know it's common in larger corporations but how about the little startups?

As far as I know there isn't much paperwork apart from a letter of employment a company has to do for something like a TN Visa but is there any other hidden ramifications or reasons why a smaller U.S company wouldn't hire someone from outside of the U.S?