My snakes go outside all the time. Well, when New England weather allows. Check out my page for plenty of pics.
It's very important to assess the area first. Make sure no pesticides are used on grass or plants. Make sure no other animals are around. We have tons of Hawks here, so I check the sky frequently.
Next, make sure there's no escape routes. Trees can be tricky, but BPs don't tend to climb, but still be very careful.
I'm also never more than arms reach away. If they wander I follow close behind.
As mentioned above, snake mites shouldn't be an issue since they're not native. But always give a good look over the snake before putting him/her back in the enclosure.
As long as you're smart and careful you can bring your snakes outside without worry.