難しい

difficult/hard/challenging

challenging

もしやってみれば何かいい結果が待っている・能力が試される

My job is challenging.

Working with kids can be challenging, but it's very fulfilling.

Rugby is one of the most physically challenging sports in the world.

Running a marathon is both physically and mentally challenging.

At 220 beats per minute, this song is really challenging to play on drums.

The reason this game is so enjoyable is that it hits a sweet spot between fun and challenging.

Chess is a very challenging game for young children to learn.

rewarding

満足感が得られる・割に合う・利益が得られる

My job is rewarding.