Sure, you pointed out some potential dangers caused by flies and not their benefits (for us). Yes, if flies get in touch with dirt, produced and distributed by us humans, they can spread some deseases.
However, I am talking about the benefits of flies and not the dangers (for that I can make another comment).
The reason why I emphasize the benefits is to balance the over all impression (especially as this community is called "fascinating" (not "dangerous") insects. :)
By the way, the by far most dangerous animal on this plantet is called Homo sapiens which destroys the rain forests, pollutes the air, throws plastic into the oceans and leads wars.