Regular hospital saves a fixed amount of armies and double that amount within 4 territories of the hospital. Field hospital lists a per cent and only talks about 'neighboring' trerritories. Why would anyone waste a slot just to use it once a day to save less than a hundredth of one or two territories?

What is a field hospital and where to find one?

It is created by an artifact.

it' probably just depends on the rarity of the artifact. if you got a higher tier one it probably would be good.

My question is does the effects stack?

If you are talking about with other hospitals and joint strike, I don’t see why it wouldn’t

My question is does the effects stack?

it would be nice if it stacked, a -3% on 4 territories is nice, but a (-3+-3+-3+-3)% would be a lot better