Honestly my main issue with the harpoon is that you lose out on a lot of damage or disable power in the arc it is in, in exchange for... keeping your enemy in that arc. For many ships, that tradeoff isn't very good.
Goldfish: Only one light gun per side. Using a harpoon makes it harder to bring your front gun to bear.
Pyra: 50% loss of damage wherever the harpoon is located., which is huge considering how poorly you turn. It is useful for preventing flanks if you hook someone with the rear port gun.
Junker: Junker turning is already good, and junkers have two decent broadsides and a fairly small blindspot. Munkers might be able to use it better than a standard junker. (throw mines, pull enemies into mines?)
Spire: Spire's have good turn accelleration. There is some argument for it as a fourth (pilot) gun, but overall I'm not sold on how effective it would be. (This might be worth testing. Use a harpoon instead of flying as pilot to keep an enemy in your front arc?)
Mobula: Mobulas have a fairly consistant trifecta, but you lose 33% of your firepower using it. MIght be useful as a top deck gun in a brawling build (pilot gun?)
Galleon: Probably the most popular harpoon ship. The left light gun can be used to keep from being flanked and to pull enemies into the heavy gun arcs. Still questionable when compared to a double hwacha+gat. Might be decent with a Minotaur+Hwacha or Mino+H.Flank combo with really excellent crew. (Hook them, turn them away, then finish with the explosive hits?)
Squid : Not useful on the front or left side guns. Could be useful as a rear gun to disrupt ships as you fly past. Potential use with a flamer - phoboes - harpoon loadout? (flame on driveby, mine on near side of enemy, yank them into the mine?)
Overall, the low damage, difficulty of use, and poor control options still make the Harpoon a tough sell for me.
