Due to the hinge action and the port shape, the correction bit is typically used to put a stop on a horse and to give him a head carriage. Depending on the height of the port you can easily contact the roof of his mouth and in all cases, as you engage the bit, his tongue will work into the port.
All bits can be severe and all bits can be mild. It all depends on the hands of the rider and his/her skill and knowledge of the way the bit works. Someone who asks about a bit typically does not know how it works and a bit like the correction bit can be severe in their hands. A skilled horseman can use a spade bit with such precision that their hand movement can be undetected but in the hands of a horse that is not accustom to one and/or in the hands of a novice to that bit, it can be torture.
So, the bit itself is not severe in the right hands but the rider needs to be trained in it's use. It is for those reasons that I prefer not to recommend that bit to someone who does not understand it.