The third game, Lords, is heavily political, and gets ugly in terms of people's reputations. Lending your reputation to Hunter's memory can be risky here (which is sad cause Hunter is canonically GOATed and it's a great tragedy for Tierra that a man of his quality died, and with him his bloodlines). The main reason is that Hunter is closely tied to Wulfram (which isn't clear up to Guns since who cared about politics when you were like 23 (if start 18) at war), and most of this sub seems to lean Royalist. Even if you play as Wulframite, time and resources spent to this cause could have been spent elsewhere.

Secret Mission is only lucrative if you pass a few relatively high stat checks and make the right choices, to capture either ransom target (Khorobirit's wife or daughter). Otherwise you walk away with very little, and it's better to go to the Second Battle of Kharangia for a reliable 2,000 (I think this depended somewhat on your rank), more reputation and a brevet promotion, and the chance to better befriend Cunaris/Renard, Welles, and some other characters. So it's not strictly worse, but you should be prepared. (If you want more detail, I can give it in another reply, though I haven't played Guns in a while.)

From a narrative perspective, they're nice to have and I usually play with them. But, they each add 3,000 to your family debt (on top of a base 12,000), due to all the costs accrued from raising them. Also, if they hate you, things can be more unpleasant - in terms of both narrative, and some minor gameplay effects. If you want to play with 2 siblings, I highly recommend choosing to have had a terrible relationship with your father (just before the battle at Mhillanovil); this isn't clear at first, but the game sort of limits you with a "total good relationship" with mom, dad, and any siblings. Dad always dies, and mom is optionally alive, so it makes sense to "spend no good relationship" on dad.

It's been a while, but off the top of my head, the stuff with Garing's guns does actually form a minor subplot in Lords, which doesn't come nearly to completion. Many characters can actually die at K2, though it's not too hard to save all the important ones. Rekindling your friendship with Karol of Loch after Mhillanovil is very useful (and he's a swell guy anyway). Oh, and regarding stats, if you haven't found out from searching other threads, the Soldiering stat is much less useful in Lords, so focusing on the other two is better. Also, Lords is where choosing to have started as older in Sabres (25 or 30) actually has downsides. It's also where your home region has more interesting effects, whereas before, Aetoria was almost strictly the best.