Learn English – Word to describe something that focusses so ostentatiously on its purpose that it fails to achieve its goal

Besides Stoney's suggestion of futile and your own implicit ones of unrewarding and unrecognised, I'd also consider unrequited which can be employed in the sense of not getting back what you've put in ...

So much of his time and effort went into practising the piano. He was a mildly depressed person, knowing at the back of his mind that a career in classical music is always difficult and often unrequited.

Unrequited:

Unanswered; not returned; not reciprocated; not repaid.

1872, Mark Twain, Roughing It, ch. 61,
One of my comrades there—another of those victims of eighteen years of unrequited toil and blighted hopes—was one of the gentlest spirits that ever bore its patient cross in a weary exile.

Optimized, from

optimal : The best, most favourable or desirable, especially under some restriction.