Things to worry about...
Pesticides, herbicides, fertilizers, parasites, neighbors who hate snakes, roaming dogs or cats, your attention wandering just long enough to lose him, him finding a small hole to slide into(mole hole? pipe?).

Also, snakes don't necasarily 'enjoy' going out into a strange open brightly lit environment to roam around. It could very well be stressful to him as he thinks something might be about to eat him, or a bird might swoop down to snatch him up, we have to remember they have teeeny brains and instinct for them is to stay hidden and safe.

All that aside.. if you think you can guarentee to your satisfaction that the area is safe from hazards and you can oversee it properly during the time out and that you can reasonably judge stress for your snake, it's your choice whether to take him outside or not.

I've taken my snakes out for photo ops in sunlight. I've also taken some out a long time ago to let them roam, thinking that perhaps they would enjoy it. After doing this a couple times I decided they were just trying to find a spot to hide, and stopped bringing them out. When I take one out for photos, I give them a quick wash afterwards just in case some sort of spray might have drifted over to the yard from a neighbor.