For the TN-1 visa, these "weird grumpy people" may, during any one of those visits, choose to deny you re-entry, completely at their whim. And there's nothing you can do about it if they do.
The scariest part is when your applying for the TN visa at the border. Make sure your hiring company lawyers draft the letter properly and possibly bring proof it's a real company if it's a new startup and so on. Don't do it yourself. Don't act like your trying to commit fraud with them because your nervous, and stick to whatever your paperwork says. If it says 'computer systems analyst', your job is a computer systems analyst. Not lead CSA, just CSA. Same with 'graphic designer', 'accountant' and so on.