The problem is that no one actually really follows the specs (or the specs don't define everything). So you can't just build to the spec and have your game work. You have to know all the ways the different hardware manufacturers cheat and adapt your stuff to their drivers.
If intel still has issues in their drivers and implementation, developing to run correctly on their cards isn't trivial at all. It should still mostly work, but it's hard to catch every edge case without experience with how they do things.