Branching code is hard.  Well, branching isn't hard. Just right click in some GUI or or type "git branch" and off you go. The actual branching of code is pretty simple and painless. In some sense, branching might be too simple in today's tools as developers are almost encouraged to branch in a VCS and "test something" quickly without a lot of thought.

Even merging has become easier, with tools to visual code changes, and automatic merging in many cases. Despite the hassles of merging, it often isn't too difficult, which can mask the complexity of branching.

However branching can easily devolve into a nightmare of multiple versions of code, with various developers unsure of the impact of their changes on a merged code base. This is especially true in a large code base where many developers each have their own branch where they work. With pressure from multiple individuals to patch bugs, add features, and streamline performance, I've met developers that end up building changes in three or four branches at the same time.

Merging all the work together, into the correct main branch, becomes tricky.  We're all human, and I've seen developers merge a change into the wrong branch. Tools try to limit this, but it still happens. The more branches that are in use, the more time that ends up getting spent on the administrative action of reconciling code differences rather than building new code (or fixing old code).

Understanding our code is hard enough, without adding the additional complexity of versioning multiple files and objects. My philosophy has always been to limit branches as much as possible, usually only allowing one, or at most, two, per team. We have also endeavored to merge back changes every few days, just to limit the amount of time spent performing merge reconciliations. I'd rather have developers have the pain of merging daily on a shared branch than dealing with branch merges less frequently (usually more complex merges).

We'll always have mistakes made, but using automated testing and continuous integration builds on all branches can help limit the number of obvious mistakes that break code. This won't prevent bugs, but our tests can help reduce the number of bugs, especially regression bugs, over time. At least, they can if we use testing on all branches, all the time.