Firstly, it is only interested in biological connections, namely the parent/child links. Any other associations are treated as arbitrary events in a persons life.
Secondly, it follows on from this that social conventions are not enforced are replicated. Conventions vary from place to place and person to person.
Thirdly, it is assumed that data will be incomplete. Virtually nothing is mandatory.
Fourthly, it is people and event -oriented. There is no reference to families as such. But aren't families the cornerstone of life as we know it? No, they are a convention, they come and go and take different forms
Finally there is the question...
If you follow both parental lines, as most do, then you find the number of people multiplying quite rapidly. My mother has three siblings, my father has eight. The tree would be incomplete without them, but what of their children, what of their spouses?. Then going back a generation, what of my grandparents siblings and so on.
I think that people are often unaware of just how big their family tree can be. A few simple sums to illustrate the problem:-
If we assume that a generation is 25 years, then 32 generations is equivalent to 800 years, a mere 800 years ago. If we round that up to 1000 years, then back at the time of Christ, my family tree has exploded to 16 billion billion people - 16,000,000,000,000,000,000 people.
Obviously there has been some in-breeding, most of it very indirect and remote, but overall enough to prune the numbers to something more sensible. Nonetheless these figures indicate just how big a family tree can be, and that is before we start back down the generations, counting up all the children. It is my guess that most people are related to each other somehow, and without having to go all the way back to the beginning of life.
I have anticipated that people will create relatively limited family trees, if nothing else the available recordds will limit the scope, but will create several for the different branches of the family and those of spouses. The family tree compiler can then cross link the people in the trees to show marriage and parental links and so on.