They feel too convenient, you need to come up some way in which they accomplish their goal that creates complications that ate interesting to work around and interesting for an enemy to try to exploit.
Well, there's already the issue of them needing something worth Resources 2-3 that was "broken by the hand of another" to create more of their kind. As long as they're properly bound, this generally means they'll try to scavenge shinies from dead enemies, or steal from people their summoner doesn't consider important and then arrange for the stolen goods to be stepped on or run over or otherwise destroyed. Then there's the preoccupation with firedust weaponry, so good luck keeping them out of your stockpiles if they manage to scrounge up a flame piece or firewand*.
Then there's the dietary predilection for things that are spoiled, broken, ruined, or otherwise "waste material", which results in things like them hanging meat from trees to spoil, making porridge out of fireplace ashes and used hookah tobacco, and generally making up for the lack of effort needed to keep them fed by creating all-new issues as they steal shit and try to encourage wastage so they can have 'good' meals. Again, the unbound ones are worse, so they end up causing even more havoc unless you pay them to exercise basic restraint instead of, say, raiding the local brewery and stealing all the unfinished alcohol so they can drink the half-fermented mash and dump out the rest.
How it's supposed to work is that
when properly bound, they're mostly troublesome in the way all demons are troublesome (they don't prioritize the way humans do, they have odd tastes, they don't really grok mortal morality), but with the added issue that since they're Conviction 4/Temperance 1 and have 9 Willpower, it's easy for things to spiral out of control if you don't constantly work to maintain an accurate picture of how/what they're doing. Their psyches are strong but brittle, so when they go into Limit Break or otherwise lose control, they
really lose control, and if they aren't bound to you that can end up with them deciding they'd rather go somewhere else, kill you, and/or take all your shit.
Adding Valor 3 to the mix further complicates matters, undermining their intended role as sneaky backstabbers and guerrilla fighters by making them crave direct confrontation and chafe at having to ignore an enemy's challenge. Again, their Willpower is beefy enough for them to soak the hit... unless other factors have been draining it away and they eventually fail the roll to suppress Valor.
In theory, they're a powerful asset at the cost of being high-maintenance and finicky. You can totally summon just one of the things and let it brew up a whole tribe of new demons, but now you're holding the tiger by the tail and they're stealing peoples' shit to make more and just generally being a nuisance and oh
my that escalated quickly, where'd that Wyld Hunt come from?
So those who can summon and bind a handful to mitigate the "runaway tribe" problem. They give the things a space of their own where they can fuck around without troubling anyone else. They get local villages to hand over their garbage so their demonic VC squad can eat like kings and replenish Willpower. They have a trusted aide or two act as a liaison between the demons and whatever other forces are at their disposal, and most of all they pray their enemies haven't read up on their demonology and know to leave sumptuous garbage-feasts out in the open to force Temperance checks from their high-maintenance scouts and have soldiers call out challenges to force Valor checks and just generally work to counter all the fretting they've had to do to keep things on an even keel.
I can't let another FCD die on the operating table -
@EarthScorpion,
@ManusDomine,
@Aleph,
@TenfoldShields, HELP!
* Essentially, they're 'supposed' to be used in either the East, where they can take the VC comparison to its logical conclusion, or the West, where you can have them running around covered in pistols flame pieces and being weird demonic monkey-pirates.