The Harpoon is slow and as such will get ripped apart by modern defensive systems. I love using it though. When we get the terminal pop up feature and other deployment methods, it will be even more interesting to use.
If you're after a tactic to get it to nail a modern ship I would try swarming it from multiple directions. Let's be honest no one is ever going to take on a ship with a couple of old missiles on there own.
Also, take into account when setting up the bearing. It's relative to the waypoint and not your direction.