My coworkers are Belgian and fly into DTW to attend meetings in Sarnia, Ontario. Their experiences rate the CBSA a little worse than the CBP, but that's just a couple of data points at 1 border crossing.
My worst border crossing was travelling with my two obviously Hispanic descended (but American) friends circa 1998 or so. We were stationed in Albany, NY and took a weekend trip to Montreal (due in part to our being under 21). Ended up in secondary heading up north for a good 2 hours, and heading back south we were informed by the CBP officer that our military ID's were not evidence of citizenship and therefore he was not obligated to let us in. So, I guess we should have travelled with our birth certificates or passports (we did not have passports, nor were they required at that time for US/Canadian border crossings). He eventually let us through.
It was at that time I learned that: (1) As a 20 year old active duty Navy Petty Officer, the only country I couldn't drink in was the one I was defending. (2) The ID that clearly states that I volunteer to die defending a country, doesn't in and of itself give me the right to enter said country.