What's wrong with using tin in medicinal chemistry?
Organotin compounds are rather toxic.  They are also persistent in the environment and have a long biological half-life.

The problem is that trialkyltin byproducts from your reaction are difficult to separate from the product.  In the lab, this is painful and (usually) involves multiple columns, but when you want to get a drug past the FDA into the clinic you have to meet purity standards for heavy metal contamination.  To a medicinal chemist, a transformation that won't make it into pilot plant and production is useless.