If anything , the Canadian border is more likely to be a crossing point for an ISIS terrorist .
The US is far safer from external tacks than other countries such as the EU countries who have huge intelligence gaps. Interpol which provides anti-terrorist coordination between countries is not near as effective as Homeland Security. The many law enforcement agencies simply do not share information the way they do in the US. Also, the government agencies within the same country often work at cross purposes.
Unlike the US, most European boarders are open, allowing terrorist free movement between countries. Although European countries have strong gun control law, the millions of weapons from Balkans conflicts are transported easily across boarders.
The greatest terrorist threat both in the Europe and the US are not external but internal. Although almost all recent US Islamic terrorist attacks are internal, such attacks are less common in the US than Europe partially because we have have 1/4 the number of Muslims in the US.