Hi there,
My piggies and rabbits have a free roam of my yard. It is enclosed so nothing else can get in, and we don't have birds of prey etc. I have been doing it now for over 10 years and have never had a problem. They live in a large shed with run attached, which they can access at all times (I see it as the equivalent of a very large hutch with the shed as the bed and pen as the bit that would be wired on a hutch. I open the pen in the mornings so that if they want to explore the yard they can. I don't open it on very cold days or when it is very wet, but on those days they don't come out of their shed anyway, they are sensible and stay in the warm.
I guess it depends on your piggies, but mine always put themselves away, they only tend to roam for 20 minute slots anyway throughout the day, they don't stay out for very long periods of time.
I've found they really enjoy it, when my back door is open they wander in and start rummaging in the boxes of guinea pig food!
I'd only recommend it if you garden is extremely secure though, as piggies are good at finding ways out.