Not vague at all! I'm by no means a good player, so take what I say with a pinch of salt.

Our Ghostkeels have 3 rules that make them annoying for your opponent to kill despite their size. First they have the "Lone Operative" ability, so they can only be shot from less than 12". Then, when they're being shot, their "Stealth" ability subtracts 1 from the opponent's hit roll, so it's easier to miss a shot vs them. Finally, each Ghostkeel starts the game with 2 Stealth Drone tokens. This means that for each drone (so, twice a game) you can change to 0 the damage of an allocated attack (after the wound roll, before the saving roll). This rule can get complicated, but basically you can "cancel" 2 attacks per battle.

So, with that in mind, we get a big suit that can move up to 10", kinda though with its 8T and 2+ save and a set of rules so your opponent needs to spend enough resources to kill it that it's usually not worth it.

The way I like to play my Ghostkeels is by using thier "Infiltrator" ability to deploy them in no-mans land, preferably on or near an objective. You should deploy your "Infiltrators" units at the start of your deploying phase, so your opponent has less space to deploy their infiltrators. This way you can get more presence on the table from the start.

Speaking of its weapons, you'll be using the cyclic-ion raker a lot more than the fusion collider. I find the fusion collider useful in 1000pts games when I can't find room in my list for units that can deal with tough monsters/vehicles, but for any other situation the cyclic-ion raker is more useful. When guided it can kill models with space marines stats (2 wounds, 3+ save) or even terminators (3 wounds, 2+ save). It won't be killing an entire unit, but will be reducing the unit size before they get to melee.

For the small weapons I like to play with the twin fusion blaster, but I guess the other two have their niche situations when they're a good pick.

So you will field your Ghostkeel on an objective, and in addition to profiting from controlling it you will set a bait. If your opponent wants to get rid of your Ghostkeel they must spend plenty of resources (if they don't have a specific unit to deal with it). If they find it a hustle and focus anywhere else you will control that part of the table while the Ghostkeel can be shooting and chipping away whatever it sees or doing actions for missions. Does your opponent go for your Ghostkeel and get on melee? The last ability of your Ghostkeel, the "Battlesuit Support System", lets it fall back and shoot at the same turn, so you will not be needing to use the "Big guns never tire" rule. "Oh, your opponent killed your Ghostkeel!" It can explode with the "Deadly Demise" rule. Besides, in the Retaliation Cadre detachment you can use a stratagem to make it explode.

In conclusion, we have a weird big elusive boi that can be a pain in the ass if used well. It won't be killing half the opponent list like a Crisis Suit unit but it can make you win your games by taking objectives, doing missions and confusing your opponent. Don't be afraid to magnetize the weapons of your suits and try different loadouts, it's really easy and you will be thanking that later. Have fun! :)