I'm currently playing through Sacred Stones Hard Mode and aiming for 0 deaths, which measn I've had to reset a lot. Sacred Stones is one of those games where if you challenge yourself to complete it properly, you *will* find it much more challenging than just reaching the basic requirements for victory. I found that if you made a mistake (like recklessly moving an injured unit into the range of an enemy) the game doesn't hesitate to punish you for it. Dealing with the reinforcements on most maps also forces you to play as efficiently as possible, or you'll lose a unit.
It's up to you whether you abuse arenas and the tower of Valni, and if you keep that to a minimum, hard mode is genuinely difficult. Chapter 6 in particular forced me to reset way too many times, even when I had Moulder use the Torch and made Seth charge at the boss. I ultimately had to get lucky with RNG to win the battle before the Bael ate the villagers.
Most people seriously downplay the difficulty of FE8's hard mode in my opinion.