This is a nice effort at a list and it is great you put the time in to compiling it. I think newer players will generally have the kinds of experience you seem to have had seriouschess, but I also think your ratings are very much a reflection of the learning curves of the weapons you are listing, the opposition you are facing and your understanding of how to use these weapons. Here are a few responses to your observations.
Banshee
Run with a couple of very skilled engineers and I promise the banshee will seem less powerful. Since the last patch it is a useful gun, but they are all useful (I've even managed to get the darn harpoon to 'work' sometimes on a ramming pyra, and especially on a squid or galleon, although it is frankly broken). I use a banshee as a backup weapon for additional killing power or to start sowing chaos after my primary attack has disrupted chem cycles. Fire doesn't scare vets.
Flamer
Weak DPS and largely useless against a properly chemmed ship. I bring this with a weapon that disrupts chem cycles (barking dog is good for this), against people who bring only fire extinguishers, or as an alternative to the Banshee.
Hades
Yup.
Mortar
Yup.
Gat
The tracer bug (tracers don't go where bullets go) makes this surprisingly hard for new players to hit with at range when the ship is moving. Your summary is otherwise good but I would add that it can effectively disable too, especially nice juicy targets like the front gun on a fish.
Hwacha
Yup.
Light Flak
I would move this down my list. It pairs nicely with a hades, less well with a gat or merc. I'd usually have another hades with a hades, a art with a merc and a mortar, banshee or carronade with a gat. I really dislike this weapon since a mortar is better if you move a little closer, and there are more versatile sources of ranged explosive damage. It functions, but I don't use it much.
Artemis
One burst artemis in the hands of a good gunner can lock down the side guns of a galleon indefinitely. Two can lock down an entire ship, any ship. Once the enemy is unable to move turn a gat on them and break their hull. As you say the weakness here is kill time, but if you are in no danger it makes little difference.
Flare
Utility, yup. Don't underestimate the chaos 10 stacks of fire can cause though. I've had aware main engineers overheat guns and save ships with this thing, and 10 stacks on a fish balloon will pull one engineer away from the hull (or if not one ship into the ground eventually).
Lumberjack
You say it is the only heavy gun you can snipe with, a good gunner can snipe with a hwacha. The lumberjack is considered one of the harder guns to use so from the sounds of things you have had some good experiences with it, this is not typical. If you are either gifted with it or take time to learn to use it it is very effective though. Tactically a lumberjack is awesome because it allows you to shape an engagement from afar. Galleon threatening your ally? Tell them to gain altitude and pop the balloon. One of my favourite weapons but the learning curve is steep. For up close damage remember lochnager.
Mine Launcher
I'm biased, I love the mine launcher. One of my crew once suggested that if there was a ship with only one light gun slot on it I'd put a mine launcher there. In skilled hands and in CQC this weapons is terrifying. It will knock enemy gun arcs off when it hits. It restricts enemy movement. It does serious damage. Spend time in the blast yard learning when and where these things deploy and how to manage ranges (the pilot has to do this by the way, making this a weapon dependent on the entire crew being skilled) and you can make life miserable. On crazy king watch the routes people take between points and mine them as you go, it is surprising how many people will crash into them. And if you see a skilled crew with a junker check if they have mine launchers. More than 2, bring something like double hades because that will not be a ship you want to get close to.
Hellhound
I suggest you search the forums for guides on how to use a blenderfish. This gun is the bane of junker and galleon pilots everywhere, putting them on the deck and slowly grinding them into dust.
Typhon
A single shot from the typhon with lochnager will typically kill any ship except a galleon. If you ever see a spire with 3 armour stripping light guns and a typhon do not get in front of it. Even without loch once an enemy hull breaks the typhon will do massive damage in the right hands. It is a hard gun to use however.
Mercury
Merc is a very effective sniper, but requires the pilot to help you aim. It can one shot most components (say the front gun on a fish or the gat on a meta), and will strip armor at distance. You suggest replacing with an artemis but consider a pyra which has an artemis in the top left. Putting a second artemis on the ship will not break the enemy hull as you observe. You could put a hades there, but say the enemy brought a double hwacha galleon. You might decide prioritizing component damage was vitally important. The obvious choice would therefore be a mercury.
Light Cannonade
That this gun appears this low in your list is somewhat sad to me, because it is a very, very effective CQC tool. A single clip of heavy at intermediate ranges will pop a balloon, and few things are more effective at preventing a squid escaping like a few greased rounds at their engines. A good gunner can pop balloons on any ship at almost any angle with this gun and it can be used to quickly snipe out enemy guns.
Javelin
Not totally useless as some think, but the physics for it is largely broken.
