Every year more lawsuits are initiated against large and giant breed dogs such as German Shepherds, Dobermans, Pit Bulls, and the Bullmastiffs. Many of these lawsuits are against the dog because of a dog bite or attack on a person, or something similar. In other cases, there is some little infraction that would have gone unnoticed had it involved a smaller and less notorious breed of dog.
Certain breeds of dogs have reputations as being aggressive and the public is less tolerant of these breeds. Much legislature has been started to ban breeding of certain breeds of dogs and the Bullmastiff is one of them. A small dog may growl at or bite your neighbor and they will get over it quickly without any trouble. However, if your Bullmastiff bites that same neighbor you may have a lawsuit on your hands. Some of this is due to the dog, but a lot of it stems from irresponsible owners that don't have their dogs properly trained.
In the case of the Pit Bulls, many irresponsible breeders were intentionally breeding and training them to be aggressive and mean for dogfights and as attack dogs. Due to behavior like this, the breed got a very negative reputation with the public and the media. This made it very difficult for responsible Pit Bull owners to have this dog they loved so much, regardless of how well they had him trained. This is the situation with many of the large breed dogs that have been used as guard dogs or excellent watchdogs.
In certain areas of the country, different breeds have been banned. If people are found to own any of the banned breeds, they will be ticketed and the dog will be removed. Bullmastiffs are on the verge of being banned in certain areas. They are a lovable excellent family pet, but the difference between them being a well behaved watchdog or an aggressive hazard to the neighborhood is a matter of how good their training is - or not. Because of this, many good dogs are being targeted purely because of their breed.
Homeowner insurance policies are clamping down on their policyholders and refusing policies to individuals that have certain breeds such as this. Some insurance companies will cover you with this type of dog, but will charge you a higher premium for the liability. More lenient insurance carriers will cover you but if your dog bites someone, they will drop you from their policy. Often they will give you the choice to get rid of the dog or switch companies.
The legal system has also stepped in with laws regarding these big dogs and dog bites. In many areas, the dog must be put to sleep if he bites someone once. In other areas, that breed of dog may not be allowed at all. Lawsuits are very prevalent in cases of dog bites.
These are all things that should be considered when you are thinking of getting a Bullmastiff. As wonderful of a dog as they are, they carry with them a bad reputation that resulted from a few bad dogs or irresponsible owners.
There are legal liabilities to consider. Bullmastiffs may be targeted for "banning" in certain areas, or refusal of homeowner insurance policies. Your friends and neighbors may be uncomfortable around this breed. In this day and age, the legal liabilities of owning any breed that looks intimidating and has a history as a guard dog should be seriously considered. People are quicker to sue if such a dog does anything even remotely questionable.