The thing about moles is that if they are cancerous it's usually melanoma. This is a particularly invasive skin cancer and can be quite dangerous if it's not caught early.
EllanVannin
You're quite correct in that BCCs are usually quite slow-growing. They can cause damage to local tissue but don't usually spread beyond the skin. Squamous cell carcinoma, the other form of skin cancer, is similar in that it doesn't usually spread beyond the skin.
The general recommendation is to watch for ABCDE with moles:
Asymmetrical
Borders are irregular
Colour (blue, red, more than one colour)
Diameter greater than a pencil eraser
Elevated off the skin
If you see any of these, you should go see your provider and have it checked out.