They would definately need protected in the garden ( I know some people will free range their guinea-pigs and the ones I had as a child were but I wouldn't let my currant ones do it)
Foxes. Cats, Magpies, Birds of Prey , Dogs.
Even in the run our little black boar used to get harrassed by magpies
. He did insist on larding up at the bars though 
Damp, Draughts, Extremes of hot/cold
They cannot vomit so are vunerable if they eat certain plants
They need to eat pretty constantly or they get gut stasis (and eventual liver failure)
They need company. We had a bereaved GP1 (when GP2 died) and a bereaved GP3 (when GP1 died). They were very meh alone (for 6 days until we could arrange new guinea-friends)
Mine come in for winter nights, out in their Pighouse by day. In winter I worry about them being warm enough and a constant temperature.
In summer, I worry about them overheating.
I can put them in their run if I'm indoors but I wouldn't leave them out if I'm out of the house.
I drive an hour to the vet which has a Guinea-Pig Clinic and a Vet with an interest in guinea-pigs (luckily they haven't had any problems really. GP3 was neutered which was a huge decision to make)
Their diet allows them to have a wide range of veg/fruit, my fridge is full of guinea-pig food. My shopping gets commented on in Sainsburys "What do you cook with all that parsley? Mmm sweetcorn, lovely"
But they are my good vegetable eating babies 
They are also quite allergenic (can be the hay) but DD is fine with smooth or abby fur, allergic to the rex fur.