I think that some grafts that are very difficult to do could have a better chance of success using an approach graft because both the rootstock and the scion are living. But the compatibility issue will be the same.
Peter
Agree the incompatibility will be the same, but the time frame is longer, so the graft won't die as quickly as in the normal detached methods.
There is more chance of success with time.
Apart from approach grafting, Inarching is a similar process, using another rootstock to support a struggling tree with root rot or graft union incompatibility.
In Citrus when distant species are used as rootstocks, sometimes leaving a few branches live on the rootstock section, is known to keep the rootstock functioning better long term. This would have to be watched closely. In normal Citrus grafting letting rootstock suckers take over can cause graft decline.
In nature there are numerous cases of live trees of very distant species grafting together where branches cross and making strong unions.
The roots of both trees remain alive and the two trees grow connected for many years.
Underground root grafting is similarly common but unobserved. Possibly more common than suspected.
Recent work suggests the underground root linkage is a highly complex chemical feedback communication system.