Well to answer your question, yes, these breeds are not recommended for novice owners. This unfortunately, is due to the fact that with these breeds there is a higher possibility that a puppy might turn out to be aggressive and bite other dogs/ humans when it grows up. This is because many of these breeds are specifically being bred for their aggression (the more aggressive dogs are being mated with each other) by unscrupulous breeders. Sadly, this gives the whole breed a bad name, even though there are many loving specimens of the breeds out there. It is for this reason that some of these breeds have either been banned in the US and some European countries.
You are partly right that the behaviour of the dog is partly determined by the environment in which it is raised. But there is a part which is genetically determined as well (similar to the way in which we inherit some of our personality traits from each parent). So, if an owner picks up a puppy that has the tendency to be aggressive, an owner who is used to handling dogs will be better at correcting/handling the behaviour and is less likely to make the problem worse or get attacked. (So even if the genes are not great, the environment is good.)
That being said, because of bad breeding, I am increasingly coming across aggressive dogs from non aggressive breeds like Labradors, Cocker Spaniels etc. This is due to the fact that many breeders breed for looks/ physical characteristics but don't pay much attention to the personality of the dogs that they breed from. So my advice would be to please check out the parents of the puppy that you intend to go for. Judge for yourself that you are happy with it's temperament, and only then commit to taking it home.
I hope this answers your question! Please do post any more questions that you might have.