Biologically speaking, you are reducing the genetic pools and the genetic diversity of genes in the family. That means that the subsequent generations of individuals to come after are going to be at a huge disadvantage due to the loss of natural selection of genes for adapting to a different environment to the ones we used to grow up in.
Socially speaking, I am more on the against side as some families are backward minded (no offence given to anyone here and these are just my perspectives, js) and to stay in that mindset of "Let's get our son/daughter married to my brother's/sister's son/daughter" and to actually pre-pick an individual, whom your kid may not even like nor envisaged in their dreams to be with, just pushes them to a tight-end corner and really reduces their confidence to actually go against their parents to fight for what they want in life. All this about families banging about pride and honour of marrying within a family... I totally disagree.
There are just too many disadvantages to speak of, it just outweighs the mere advantages it may have.