Both Greater and Black Demons are high-weight tasks with disappointing loot tables. Duradel often assigns large quantities of these demons (up to 200),which means a lot of time spent fighting slow,single-target monsters with RuneScape gold very little in return. On top of this,finding a world without competition can be a hassle,and the spawn rates aren't always ideal. Ironman players may consider keeping these tasks since they drop items needed for Slayer Helm upgrades or Emberlight,but for most players,these demons aren't worth the time and effort.
Spiritual Creatures

Spiritual Creatures found in the God Wars Dungeon (GWD) are notoriously annoying to deal with. These monsters have low Slayer task weights but are widely considered some of the most tedious to kill in the game. They require a significant amount of preparation,as you'll need specific God-aligned gear to avoid constant aggression from other GWD monsters. Their loot table is also poor,so the task isn't worth the hassle unless you're specifically after a pair of Dragon Boots. For me,these creatures were the first I blocked,and I've never regretted that decision.
Steel Dragons

Steel Dragons,like Black Dragons,are high-defence,high-Hitpoint monsters that can be tedious to kill. Their primary attacks include a strong dragonfire breath and powerful melee attacks,so you'll need to bring protection prayers like Protect from Melee and Piety to maximize your efficiency. Although the introduction of Project Rebalance has made Steel Dragons weak to Earth Spells,they remain a slow kill,and their loot table isn't very rewarding. Steel Dragons often come in small task sizes,and even with Earth Spells,these tasks can take far longer than preferred. Given their high difficulty and low reward,Steel Dragons are a prime candidate for blocking.
Mithril Dragons

Although Mithril Dragons are unlocked through Slayer tasks and offer good experience for higher-level players,their slow kills and low drop rates make them inefficient. They have a high defence level,making them challenging to cheap RS gold kill without using powerful weapons and high-level prayers. Given their lack of good drops and high task weights,they don't fit well into an optimized Slayer strategy.