Hard boards demand the advanced end of the technique ladder: exclusion (a star here would leave some region unable to fit its stars — so it can't go here) and under/overcounting, where you tally the stars a band of rows must hold against the regions inside it. The deductions are provable but rarely obvious.
Every hard board is still verified guess-free: our generator only publishes puzzles a logical technique ladder can finish, so if you're stuck there is always a next deduction — the strategy guide shows how to find it.