IMO.. the ideal environmental (but unrealistic) would be:
1. Year round mild climate
2. No predators of any kind
3. No dangers of a close by, busy road
3. A big, tall, open but sheltered shed/building with plenty of high roosts for them to use of they choose
4. Tall trees that provide shade and a good thick canopy year round if they choose to roost there instead
5. Safe/sheltered areas on the ground for the Guinea Hens to lay and brood their clutches
6. Plenty of bugs, greens and seeds to eat year round
7. Constant source of fresh water
Protection from predators would be the biggest issue... all the conditions I listed above are also ideal for a lot of the predators that do the most damage to free range flocks. And even if the ground predators are minimal, the owls and hawks with make up the difference
ETA:
The heartiest Guineas are usually the common Pearl Grey Helmeted Guinea Fowl