On the contrary, I'd argue that it's not true in specific cases.
"Because it's hard" is a cop-out.
"It's too hard to accomplish given constraint [X]" where X is a deadline, financial constraints, or other real/tangible resource limitations might be one thing. But if you're working on your own timeline on some sort of open-source project, or there is nothing external preventing you from acquiring the expertise/resources to conquer the hard problem, then "Because it's hard" is an absolutely shitty excuse to not do something.
I suggest you read "It's too hard" when written by other developers as, "It's too hard [given that I spend N hours a week on this and would rather actually accomplish something in the next two months than learn the 'right' API]." Or, "It's too hard [given various constraints that I'm not going to explain to you but are valid to me.]" It'll save you having to give speeches about shitty excuses.
That said, if it makes sense for your project, make it happen! :)
Even if the long term goal is more portable GPU support it still makes some sense to get a CUDA implementation up first if it is easier to get to. It then allows real world testing faster, can always go to openCL later once they know more.
Just out of curiosity: how often did you see that happen (not only related to GPU's, but technologie decisions overall)? In my (little) experience the change at a later moment will not happen. Most of the time because the management has a new idea/project which you have to attend to.
"Because it's hard" is a cop-out.
"It's too hard to accomplish given constraint [X]" where X is a deadline, financial constraints, or other real/tangible resource limitations might be one thing. But if you're working on your own timeline on some sort of open-source project, or there is nothing external preventing you from acquiring the expertise/resources to conquer the hard problem, then "Because it's hard" is an absolutely shitty excuse to not do something.