Can't say that I have ever heard it called a branch union before today... Difficult to cut with a chainsaw as it needs to be cut fairly accurately. Equally difficult to cut on a bandsaw if you have one that can cut that high. Do be wary of any bark inclusions, in part because they can fly apart while turning. Most of the crotch feathering is below the branch junction, so I usually don't plan on keeping the branch parts, but that can change depending on the tree. Some trees have beautiful feather patterns, some have none. Generally a tree out where it gets a lot of wind exposure will have lots of feathering. A deep woods tree can have none. Really bummed out years ago, a black walnut crotch with 18 inch diameter branches that was down in a canyon had no feathering at all, but really nice compression figuring under the branches.