As BP's husband, please let me respond to this one on her behalf
sorry to hijack, but why is that?
is there something specific about Filas that make most of them hard to train, or have agression issues?
Allow me to digress from the fila to the purebred in general before answering directly.
Purebred dogs are bread to fufill one (or more) specific functions. For example pulling sleds or carts, rescuing drowning victims, fishing, hunting big game, hunting small game, controlling the rodent population, protection, racing, finding and rescuing people trapped in snow avalanches, and working with livestock.
This means that if you have a sighthound and they see a rabbit, you know they're going to take off after it. If you have a water dog, and the kids are flailing in the pool they just might be rescued, if you have a herding dog you might loose them for hours while they run circles around a flock of gees. If you have a guarding dog they might attack someone who is threatening you or your family.
This is what these dogs have been bred for centuries to do. It's in their blood, its in their genes. You cannot train a site hound not to want to chase the rabbit, or the herding dog not to keep groups of animals in a tight bunch.
When you buy a purebred you have the chance to know about these behaviors up front. You'll know if you cant trust your dog off leash in wooded areas, or if you should expect to find holes all over the yard if you get a gopher, or you should expect your dog to notice shadows and things moving on the walls and bark at them, or if they're going to drag you for a walk if you ever put a harness on them.
That is, for the most part, a dog of a breed will do what a dog of that breed should do. You can curb, but never destroy, those instincts.
Now, let me talk about the fila before directly answering your question
The fila is a remarkable dog in that it, by necessity, is an everything dog. You see the fila (also known as a brazillian mastiff) came into being because it was needed. The people living and working in brazil at the time had the need for several different things from their dogs.
They needed a dog which would protect their estates... and those estates were sometimes vast territories. Therefore they needed a dog which was protective of its property.
They needed a dog which would assist them in hunting big game (such as jaguar) which presented a threat to their selves, livestock, and workers. Therefor they needed a cunning, resourceful dog, powerful enough and capable enough to take down prey as big or bigger than itself.
They needed a dog which would mind their livestock. Therefor a dog which would attach itself to other animals in its "family" and defent them from predators, thieves, and smart enough to recognize inherently dangerous situations.
They needed a dog which would protect their investments. This meant slaves. At the time slavery was in full swing in brazil, and a slave was a piece of property which had the will, sometimes, to liberate itself from ownership. Therefor they needed a dog which will attach itself strongly to its masters, while willing to hunt down, catch, and detain human beings.
They needed a dog which would defend its master to the death should the situation arise. Therefor they needed a dog which could, on its own, judge a situation and determine its severity without its masters help.
The fila is a dog which can make judgments all on its own. It can think all on its own. It can determine intent all on its own. It can trick, and trap. It can hunt, and it can defend. It can, and this is fairly unique, and will act even against your wishes if it feels that the situation is dire enough to warrant it.
Let me put it another way. Think of your town, and the worst part of it. The part where you never drive through after sundown and especially never walk through. That part. A fila is the one dog I would want with me most if I had to walk through that part of my town.
The flip side is that you will never ever find a dog more dedicated to its master anywhere. More than any other this is a dog who defines its master as a part of itself. Never will a companion care more about you than your fila will.
And finally there is something of a divergence in the fila world making knowing the ancestry and parents specifically worth knowing. There are two thoughts as to the "original" temperament of the breed. The first thought is that the fila should outright hate all strangers. And the second thought is that the fila should be watchful and wary but not necessarily biased towards hatred of strangers. And breeders on each side of the temperament divide are breeding for outright dislike of strangers, or simply a distrust of strangers.
Now... about mutts
When you breed a purebred dog you are doing your best to guarantee the traits both physical and mental that the dog will posses. As with anything there will be tolerances and allowances within this standard, but on the whole a random sampling of golden retrievers will all be identifiable physically and behaviorally as golden retrievers.
This is done through a perversion of natural selection. The breeder simply eliminates the genetic possibility of, lets say, a short coat or a long coated dog. The genes simply aren't there on the mother or the fathers side to produce a pup with a short coat.
But when you introduce randomness to the equation you get unpredictability. You just don't know for sure what you're getting. And you don't know in what measure.
For example Buddy is (it is largly agreed upon) a Lab/Pointer mix. But which percentage of him is lab? and which pointer? Will he behave more like a lab or a pointer around other dogs? humans? children? cats? birds? Will he be slenderer like a pointer, or less so like a lab?
You see with a mutt there are questions to which the only answer is "we'll have to wait and see"
And finally to answer your question
Do you want the predictability of a purebred, or the "wait and see" of a mutt with a dog thats been bred to think without you, track and detain humans, roam large territories, hunt large game, herd large animals, and to die for its convictions, and weights 100-160lbs?
Will your lab/fila die for you or for the ball? Will your greyhound/fila die for you or for the rabbit? Will your pure fila be of the "every stranger should be controlled" or the "alright, just don't do anything I might have to make you regret" temperament?
Wouldn't you rather know the answer to those questions with this breed, than wait and see?
Which is why we would rather stick to pure lines with the fila than unknown ancestry.
With any dog you need to understand what you're getting yourself into. But with such a dog as a fila you need to have a deep understanding of the breed, and yourself, before making that plunge.
Conclusion
I hope you understand where we're coming from on this issue. The fila is not "aggressive" by any normal measure. As a matter of fact a fila is not at all likely to chase you any distance. Once you're no longer being a threat they no longer care about you.
So it's not an issue of aggression, It's that It's that relating to a dog who has such a strong personality and awareness on so many levels is a big enough task. It's that controlling any wilful 140lb dog is a big enough task. It's that being a dominant owner without causing fear is a big enough task. It's that knowing your dog well enough to steer it clear of situations that you know won't come out OK is already a big enough task. And thats all with knowing the breed.
You don't play chicken with the train unless you're prepared for the possibility of getting hit. And thats why we buy a pure bred fila. And thats why people buy any pure bred. Because sometimes you just need to know what you're getting yourself into, and a purebred lets you make an informed decision up front.