[vaughandroid]

I would say yes they are, in general. The title of the post is an absolute, but as the author says in the second paragraph:

> As with all HeuristicRules, this is not a rule that applies 100% of the time. Code is generally clearer and easier to maintain when it does not use globals, but there are exceptions.

For me, the root of the problem with them is that they introduce coupling (or to put it another way, they break modularity). When you introduce global state, you introduce it everywhere. In reality there are probably only a handful of places that it relates to, but "is the global state involved here?" becomes a question you need to ask yourself whenever you write/update any part of the code.

The same argument can apply at more fine-grained levels. If you introduce a module-level variable, you need to keep it in mind whenever dealing with any code in that module. Similarly for class-level and instance-level variables.

There is a trade-off: ease-of-access and clarity vs the extra cognitive load you are introducing to every part of the scope. In my opinion, that balance determines whether a variable is good or bad. For any non-trivial piece of code, the sum almost always works out on the side of reducing the cognitive load.