1) Difficult 2) Fruitless 3) Like ingrates 4) Not likely to be rewarded 5) Unappreciative 6) Unfulfilling 7) Ungrateful 8) Unsatisfactory 9) Unthankful 1) Tiresome 2) Unappreciated 3) Unappreciative 4) Unenviable 5) Ungrateful 6) Ungratifying 7) Unrewarding 8) Unthankful 
• (a.) Not obtaining or deserving thanks; unacceptable; as, a thankless task. • (a.) Not acknowledging favors; not expressing thankfulness; unthankful; ungrateful.
Thank'less adjective 1. Not acknowledging favors; not expressing thankfulness; unthankful; ungrateful. « That she may feel How sharper than a serpent's tooth it is To have a thankless child!» Shak. 2. Not obtaining or deserving thanks; unacceptable; as, a 
[adj] - not likely to be rewarded
unappreciated adjective not likely to be rewarded; `grading papers is a thankless task`