Yes, certificate pinning would alert the user to a MITM attack, but it's not commonly used. By "hard to detect", I meant that it's impossible to see simply by e

Yes, certificate pinning would alert the user to a MITM attack, but it's not commonly used. By "hard to detect", I meant that it's impossible to see simply by examining the certificate if it's genuine, you can only detect when the certificate changes. And since SSL certs expire and are re-issued all the time, it makes it a fairly large headache to continually try and guess whether the other party changed their own cert or if you are experiencing a MITM attack.