Orchids, best in pots or the ground?

They grow well in either but you need loose sandy soil. Fertiliser is really important too.

What sort of Orchid! There are quite a lot.

Phalaenopsis grow best in pots, Microtis grow best in the ground.

Eh.! What about cymbidiums.!

many orchids enjoy having their roots slightly confined, in the wild they often grow in the crevice of a fork in a tree

Cymbidiums are naturally variable. Some of the Australian ones grow in trees but the roots get into the rotten wood and can run meters away from the plant. Others will grow on trees and rocks and send roots upwards to create a trap for detritus and falling leaves. Some of the Asian ones like it really moist, but most are lithophytes and epiphytes and grow on mossy branches and rocks.
Most of the hybrid ones around here will do better in a well drained (and frost free)area of the garden than they will in a tree though. They won't tolerate being sodden for any length of time.
The thing about the orchids liking it potbound is more about them not liking their mix sodden and waterlogged - a pot full of roots will quickly soak up any moisture and the mix (or what is left of it) will not remain sodden and cause the roots to die, While a plant with few roots in a pot full of wet (even bark based) mix will tend to stay wetter for longer, which can kill the remaining roots causing the plant to sulk, especially in winter when it is cold.

Point being that "Orchids" are not all the same. It's a large plant family and they all like different things!