A little process goes a long way. Process is, after all, just a manifestation of organization. It lays out an approach to some accomplishment as a breakdown of its parts. For simple goals the path may be obvious, but for highly complex things the process guides people through the confusion and keeps them from missing important aspects.
Without any process there is just disorganization. Things get done, but much is ignored or forgotten. This anti-work usually causes big problems and these feed back into the mix preventing more work from getting accomplished. A cycle ensues, which among other problems generally affects morale, since many people start sensing how historic problems are continuously repeating themselves. Things either swing entirely out of control, or wise leadership steps in with some "process" to restore the balance.
Experience with the chaotic none-process can often lead people to believe that any and all processes are a good thing. But the effectiveness of process is essentially a bell curve. On the left, with no process, the resulting work accomplished is low. As more process is added, the results get better. But there is a maximal point. A point at which the process has done all that it can, after which the results start falling again. A huge over-the-top process can easily send the results right back to where they started. So too much process is a bad thing. Often a very bad thing.
Since the intent of having a process is to apply organization to an effort, a badly thought out process defeats this goal. At its extreme, a random process for example, it is just formalized disorganization. Most bad processes are not truly random but they can be overlapping, contradictory or even have huge gaps in what they cover. These problems all help reduce the effectiveness. Enough of them can drive the results closer to being random.
Since a process is keyed to a particular set of activities or inquires, it needs to take the underlying reality into account. To do this it should be drafted from a 'bottom-up' perspective. Top-down process rules are highly unlikely to be effective primarily because they are drafted from an over-simplification of the details. This causes a mismatch between the rules and the work, enhancing the disorganization rather than fixing it.
Often bad process survives, even thrives, because its originators incorrectly claim success. A defective software development process, for instance, may appear to be reducing the overall number of bugs reaching the users, but the driving cause of the decreases might just be the throttling of the development effort. Less work gets done, thus there are less bugs created, but there is also a greater chance for upper management to claim a false victory.
It's very easy to add complexity to an existing process. It can be impossible to remove it later. As such, an overly complex process is unlikely to improve. It just gets stuck into place becoming an incentive for any good employees to leave, and then continues to stagnate over time. This can go on for decades. Thus arguing for the suitability of a process based on the fact that its been around for a long time is invalid. All it shows is that it is somewhat better than random, not that is is good or particularly useful in any way.
Bad process leaves around a lot of evidence lying around that it is bad. Often the amount of work getting accomplished is pitifully low, while the amount of useless make-work is huge. Sometimes the people stuck in the process are forced to bend the truth just to get anything done. They get caught between getting fired for getting nothing done or lying to get beyond the artificial obstacles. The division between the real work and its phantom variant required by the process manifest into a negative conflict-based culture.
For software, picking a good process is crucial. Unfortunately the currently available choices out there in the industry are all seriously lacking in their design. From experience the great processes have all been carefully homegrown and driven directly by the people most affected by them. The key has been promoting a good engineering culture that has essentially self-organized. This type of evolution has been orders of magnitude more successful than going out and hiring a bunch of management consults who slap on a pre-canned methodology and start tweaking it.
That being said, there have also been some horrific homegrown processes constructed that revel in stupid make-work and creatively kill off the ability to get anything done. Pretty much any process created by someone unqualified to do so is going to work badly. It takes a massive amount of direct experience with doing something over and over again before one can correctly take a step back and abstract out the qualities that make it successful. And abstraction itself is a difficult and rare skill, so just putting in the 10,000+ hours doesn't mean someone is qualified to organize the effort.Picking a bad process and sticking to is nearly the same as having no process. They converge on the same level of ineffectiveness.