I have to disagree. I've been running a Radeon HD 6850 for the past four years, and I've never experienced this issue or had any trouble getting games to work properly. I'm even playing Star Citizen on it, albeit on low settings.
AMD gets a lot of bad rap in these situations, and most of unfairly, from what I've seen.
Well, I've experienced trouble in many cases with ATI cards - however, I'm not referring to difficulty with commercially available games (that hasn't happened for years), but rather with games in the course of development. When working on Dogfight 1942, each time we produced a build with any new graphical features, the programmers had to produce a special alternate executable and DLLs for me because of my ATI card. It goes without saying that the final release version was able to handle both NVidia and ATI, but in development, this wasn't a given. Most of the team used NVidia cards because that's what's most common. So every new feature was coded on an NVidia, and would either work or not work on ATI, we usually wouldn't know until somebody actually tried it. And of course, our graphics programmer was very experienced, and the moment a problem came up, he would usually figure out a solution very rapidly. This is not necessarily the case with something like Saga, which relies on a technology repeatedly modified by a multitude of programmers. Some of them will be very skilled and experienced, others will be well-meaning amateurs. And then, even the skilled ones, might not necessarily have time to address issues, particularly issues that come up years after they're done.
But again, it isn't a case of ATI making bad cards. The Xbox 360 had an ATI card inside, and obviously there were no issues there. But when making a game for a console, you only ever deal with one hardware configuration, one set of drivers, one everything. On the PC... well, it's just like it is with browsers - in theory, they all support the same HTML standards, in practice... they don't. Sometimes a website will look slightly different in IE and in Chrome, Firefox, et cetera. Sometimes it won't look slightly different, but rather it will be a total mess. So, what happens then? Well, the developers pay the most attention to the most common browser on the market, and only in second place do they do their best to ensure full compatibility with other browsers. Even so, there's less potential for screw-ups with browsers than there is with graphics cards, particularly when it's not just the difference between the two companies, but also small differences in functionality between individual products (and of course, the whole point of having unified drivers, and DirectX or OpenGL and all that is to mitigate such issues - and yet, they're there).
If ATI had most of the graphics card market, I have no doubt that I would now be complaining about NVidia cards having constant compatibility issues...