Almost any process that measures human output will be gamed (optimizing for measured output rather than overall success). It's basically impossible to game sales without resorting to outright fraud, whereas there are all kinds of ways to game individual engineering output measurements, in ways that do not align with the business objective.
Sure. That's all true. But it doesn't change the situation.
There are difficulties, but there are difficulties in any job. It's still the job of management to evaluate the performance and contribution of their employees.
The more rote the process, the worse it will be, since, as you say, it will be able to be gamed. As a general rule, I've found larger organizations are worse at it because they have more rote processes, with complicated matrices, charts, etc.
I've been a cog at small, medium, and large orgs. I've been team or project-level management at small and large orgs. I was happiest being management at the small level. Much less politic-y stuff and more freedom to manage the way I saw best. I enjoyed being managed most at the mid-level company. Less room for managerial whims that small companies can suffer from. (Yes, I know I'm playing both sides on that. Heh.) And less of the 7,000 item Employee Evaluation Form Of Doom that big companies have.
It's hard, no doubt. But a manager saying they can't accurately evaluate their employees is literally saying, "I can't do my job." Any manager should always have a very, very good idea what's going on with their employees and their contributions.
Sure you can game sales to some extent. You can game the specific commission system. That's why it's very important for the specific incentives of the commission system to match the specific goals of the company.