We adopted a pair of cats we'd been told were indoor only, always had been and they hated outside. As we lived near a busy road at the time (with plans to move in future but not immediately), we were delighted that we could get the cats we wanted without feeling guilty for keeping them in.
Both of them were TERRORS - yowling all night, anxious behaviour, all the things the person we adopted them off said had put him off owning cats (poor babies) - until we moved 6 months later to a quiet culdesac with loads of green space and started letting them out. They're so much happier catching flies, climbing trees, and running all their energy off outdoors - when they come home their toys barely get a look in any more and they're both such content, relaxed cats.
Their younger sibling, however, is a lazy boy who isn't arsed - he goes out occasionally for a sniff and doesn't seem scared at all, but he is perfectly happy inside. I think if it suits the cat, then fine, but keeping cats inside for no good reason is cruel - especially as so few people seem to understand how high cats play/exercise needs are. When we had our older two indoors we would spent at least an hour a day playing with them - fetch, laser pen, fluffy snakes, proper active engaging play with new toys all the time to mix it up and keep them mentally engaged - and it still wasn't enough.