A Fila Brasileiro is one of the few breeds of large dogs raised to herd livestock. They are most comfortable in country settings and can be hard to keep in an urban environment. Since they were bred to herd and protect livestock, they have a natural aggressive tendency to anyone that that they do not consider part of their herd. This can range from other pets to strangers. They are naturally suspicious of strangers and can have an over-protective instinct that can get them in trouble in a social setting.
If you have children in the household, it is not advised that you get a Fila Brasileiro. Even though the dog will protect and love your children, and even can be very gentle with them, this does not extend to your children's playmates. They may not understand two children horse-playing and choose to intervene on their charge's behalf, potentially harming your kid's playmate. There are liability issues with owning a large dog and with the Fila Brasileiro, there are more than usual. Its size alone can cause all sorts of destructiveness to your home and possessions, not to mention guests in your home.
This dog cannot be socialized very well, like other guard dogs. It will always have a suspicious temperament, even though now there are breeders interested in breeding out some of the dog's natural aggressive tendencies. Until then, the dog should be isolated from your neighbors' yards with a high fence, with posts that are dug deep to keep it from tunneling. You can get this dog obedience training, but that doesn't assure good behavior in a social setting for this breed. Even at dog shows, the judges are instructed not to touch the dog because of its natural aggressive tendencies.
You will also have to consider that this breed is sometimes banned in different home subdivisions. You may want one, but you may not be allowed to have one. You cannot own one in the UK legally without special permission. In addition, you may have to pay more on your homeowner's insurance if you decide to own one.
If you are someone who wants a great and loyal guard dog, and lives out in the country with few visitors, this can be an ideal dog to have. Just make sure that it has a secure area to roam where wandering strangers will not disturb it. You do have to have a very firm and strong personality to own this dog as it needs to know that you are the one in control at all times and not the other way around. If you get it as a puppy, you can spend time in obedience training to make sure it understands who's the boss.