We adopted two kittens late last year, and thought it would be nice to let them outside to run around and explore nature. We used to have an indoor cat that lived in apartments his whole life, and I always felt a bit guilty since he wasn't very happy and would always try to get outside.
Our kittens learned to hunt very quickly and would bring animals into the house. Lots of lizards, mice, and birds. We got tired of burying dead animals and vacuuming up feathers. Then one of our cats got into a fight and got bitten. The vets think that the bite must have punctured a vein or artery in their shoulder, which caused a huge hematoma that swelled up their chest. They had severe anemia and almost died from internal bleeding, so they had to give them a blood transfusion. They didn't have any cat blood available, so they used dog blood instead. (Apparently that's quite common!) He's almost fully recovered now, but still has a limp that is slowly getting better. The vet bills ended up being over $6,000. Fortunately we had pet insurance, so we only had to pay $1,300.
So we don't let them outside anymore. We were surprised to see that they don't seem to mind too much. They're not begging to go outside and they still seem perfectly happy. We've put a little bit of grass on an internal balcony, and we might try to build a catio enclosure so they can still have a little area to go outside. Maybe some netting like this: https://catnets.co.nz
EDIT: Cat tax: https://imgur.com/a/umQpcvK