Angelina It depends on the material. The cage I wear all the time is made of "biosourced resin", or at least that's what the manufacturer claims. It's pretty stiff but the material itself is still "soft". In that cage I'm able to get an erection up to a certain point until my penis just goes flaccid again. It doesn't really hurt, and when it does, only for a short time. However, my "travel cage" is made of some kind of plastic and that's a completely different story. I'm able to get a little harder than in my usual cage, but it really hurts as soon as I start to get hard and doesn't stop until everything is flaccid again. I'm really glad that I almost never wear it.
As long as you don't clamp any blood vessels in the cage it's pretty safe. It only seems that the penis gets smaller after wearing for a long time (I can confirm that), but it seems like that is only temporary and is reversed after a few days of being unlocked.