Am a geneticist. Inbreeding doesn't cause harmful mutations, only concentrates them if the inbreeding stock's genomes are compromised.
R1a and b types (western and NE euros) have very "clean" genomes, given to the fewest genetic diseases, they only show up if other genes are introduced. Don't want to get too spoopy, but there are historical records that certain lines were hybridized with something not homo sapiens. Genesis 6 mentions it, books of Enoch and Jasher too. All ancients address it.
Hybridization destabilizes genomes. The same modern lines that have unstable genomes are also those that the ancients pointed to as emanating from hybridization. Coincidence?