So very true. Almost all the misery of maintaining and upgrading a Rails app comes from the interactions between plugins, each other, and Rails.
Why is this? I don't know exactly. It could be the way that plugins tend to manipulate multiple layers of the stack and tend not to use clearly-defined public interfaces. I suspect that more of it is the fact that plugins are the self-promotion tool of choice for the jobbing Rails developer: they fall from grace as quickly as they rose, abandoned by the developers to whom they gave a leg up, supplanted by a new, different plugin with an obviously - everyone agrees! - much more correct way of solving the problem. Meanwhile, your old Rails app is dependent on a dozen pieces of decrepit abandonware.
Monkeypatching! It's all fun and games until someone else decides that monkeypatching #poke is also what their gem or plugin needs. Then, of course, you start accidentally #poke'ing your Eye when you least expect it, and cannot figure out from where or why...
Why is this? I don't know exactly. It could be the way that plugins tend to manipulate multiple layers of the stack and tend not to use clearly-defined public interfaces. I suspect that more of it is the fact that plugins are the self-promotion tool of choice for the jobbing Rails developer: they fall from grace as quickly as they rose, abandoned by the developers to whom they gave a leg up, supplanted by a new, different plugin with an obviously - everyone agrees! - much more correct way of solving the problem. Meanwhile, your old Rails app is dependent on a dozen pieces of decrepit abandonware.