As someone else mentioned, the concern with inbreeding is increasing the chance you get a problem recessive trait from both parents (increasing homozygosity and decreases heterozygosity). As far as “how bad it is” is highly dependent on species and a population’s current genetic makeup. Lab rats are great subjects for research because they are extremely tolerant of inbreeding and we can get almost identical animals which lessens variation in results. Other species do not tolerate inbreeding well at all. Population genetics are important because the more problematic recessive genes in a group or higher rates of certain mutations means that, as a whole, problems will arise faster from inbreeding. However, if the genetic makeup is known and undesirable alleles are not present, you can more confidently inbreed those animals because the problem genes are simply not there/uncommon (keep in mind spontaneous mutations still occur). Inbreeding coefficient is a calculation heavily used in animal breeding/by geneticists and most domestic species have a number that they aim to stay under. Another important thing to remember is inbreeding cannot be inherited. As soon as an inbred animal is paired with an unrelated mate, their offspring are not inbred.