Why are insertion and deletion mutations so harmful?

the insertion on removal of mutations are dangerous in nature.  the process is basically just one large gamble to see what it does. and even though we have a decent understanding of the genomes of many animals and plants, we still dont know everything. so if we tamper with certain genes we may cause a evolution or we might kill the subject and the potential for it to procreate.

answer; i believe the answer is (polygenic inheritance;