Hell some US brokers won't take Canadian clients. Some kind of cross border agreement. I don't think Canadian brokers will take US clients. (Stock Brokers, not sure about futures or forex)
And that's what I see as potential problem "some sort of agreement". Between whom? Based on what regulation issued by which regulatory body? It's like saying "there is a cross border agreement to keep gasoline prices high" when in effect it could be corporations colluding and slicing and dicing up the market. But customers are told and sold the idea as if it was some sort of law or regulatory obligation.
I have no idea. All I know is that years ago, before Canada had discount brokers I opened accounts with several US discounters. They were eventually closed due to whatever regulations. TD Ameritrade was one that moved my account back across the border as TD Greenline. IB eventually opened in Canada and sovled that problem.
IB still does that (though I think it's $13K now), and occasionally raises it when too many people get liquidated because of ES volatility.
Been with AMP for a few years already and have had nothing but great service from them. They answer emails quickly and are as helpful as they can be. Account opening was easy as I had the necessary paperwork and funds. No complaints whatsoever