Of all the Devil Fruit powers in One Piece, Big Mom's ability to grant life to inanimate objects seems like it would be one of if not the most useful abilities to have. Much like the Shadow-Shadow Fruit allows the person who consumes it to transfer souls from living things into corpses or other people, the Soul-Soul Fruit allows the person to transfer souls into non-living things or animals. But the ability is a dark one, and actually using it might not make you the best person.
Carmel was the first user, but the power was passed on to series Big Bad, Big Mom, after Carmel's death. Big Mom uses her power to extract the souls of others and bring an army of "Homies" — animated objects totally loyal to her — to life. When a soul is extracted, Big Mom literally holds someone's life in her hands. She's able to decide how much of a soul she wants to take — taking out too much can result in that person's death — create incarnations of herself that are able to steal souls and use pieces of her spirit to create Homies that are much stronger than those made from the souls of others, as seen in the cases of Zeus, Prometheus and Napoleon.
The homies take on a more human appearance once the soul has been placed, and animals can gain the ability to stand upright. Inanimate objects only gain a human-like face, however. Homies are also granted the abilities and memories of the person whose soul they possess and they gain a mind of their own, albeit one dedicated to Big Mom. Objects created by other Devil Fruit are also capable of having souls inserted into them. Basically, the Soul-Soul Fruit allows the user to live out their demented Sesame Street dreams.
But there are serious drawbacks to this power. In terms of battle prowess, the fruit's powers are useless if the target has no fear of death or the user — without that, their soul can't be taken. Another drawback is that extracted souls cannot be placed into other humans or corpses. Homies that experience extreme fear can wither away and die, and they can't attack another homie that has a piece of their same soul. They're also weak against the Revive-Revive Fruit's abilities (unless the homie is created from the soul of a Revive-Revive user).
The biggest downfall of using the Soul-Soul Fruit is the simple fact that it trades in souls. The most potent Homies come from the user deploying fragments of their own soul, which means the user has to give up bits of their lifespan and humanity in exchange for power. In the hands of someone with a functioning moral compass, avoiding using others' souls in favor of their own could kill them. But, Big Mom's case, this isn't really much of an issue since she just takes the souls of others, which is a significant issue in itself. Big Mom is a monster, so it likely doesn't bother her at all, but snatching someone's soul away to do battle in your service would weigh on any decent person's mind.
But, even using it just once would put someone on par with the pirate queen — it's that bad. Then there's the matter of creating Homies. The Soul-Soul Fruit's abilities basically allow for soul transfer, rather than creation. So, you aren't giving life so much as you are taking it, brainwashing it and sealing it away elsewhere.
Taking all of this into consideration, it's clear that using the Soul-Soul Fruit's abilities is more a matter of morals than of capabilities. While the user is able to take small amounts of a person's soul at a time, stealing the very essence of someone else's life is a pretty abhorrent thing to do. Each time the user takes part of someone's soul, it detracts from that person's lifespan, adding up every time the user goes back to the well, possibly ending in the victim's death.
Of course, someone could offer a fragment of their soul of their own free will, but that wouldn't make taking it alright. The moral conundrum that is the Soul-Soul Fruit makes it difficult for anyone even a little good to justify using it but, luckily for Big Mom, there's not a good bone in her sizeable body.
