Heh, hirelings are utterly worthless, and a complete waste of money. Really, they're all like level 1, and get 2-shot by even the easiest invasion fights. You're lucky if 1/4 of them survive, even if you use crowd control. The only time you ever want to use auto-resolve is if nothing is built.
Prestige and security do modify income somewhat. Security generally lowers losses to bandits, and Prestige can increase the volume of money. Problem is, if you get to, say, 50 security and 40 prestige, you're likely to get around 800g out of it. Meanwhile, your awful hirelings have sucked down like 1200g in wages, so you're pretty much always operating at a loss. It sort of breaks even when you throw the "free" crafting materials in the mix (that you pay thousands and thousands of gold to get) - if you use that sort of thing. I suppose you might break even on the investment of the entire keep over 15 game years or so...
There are only 2 things that make the thing worthwhile. First is the Warden, which gets you the nice bounties for excellent xp and loot. Second is the occasional merchant with unique items.
That all said, yeah, the entire stronghold should be fixed up a bit. Personally I would have preferred there not be a stronghold in this iteration of the game, and save it for the sequel/expansion, as it seems a little early to be handing out titles. In NWN2 you had to basically wipe out several armies by yourself to get handed your ruin of a castle, which while a bit excessive, seems more reasonable than this. Otherwise, with the limited funds and time they had to throw at the project, it fell short of pretty much every stronghold in any major title in the past 17 years. Good ideas - just way too rushed.
Hope they go back through - spruce things up a bit. Tie it in with the story or at least the world in general (it's really odd that nobody in the game even knows that you're the lord of Caed Nua). Add in some surrounding lands, so that you can hunt down some of the troublesome bandits, solve some local problems. Make judgments, foster trade deals. Contend with the bridge down South being restored (which threatens your trade). Set up your own little colonial initiative like Raedric. That sort of thing.