Their drop table is poor overall,with most of their loot being either low-value or rare drops like the Ensouled Head or Smoldering Stone. You will almost always lose money on RuneScape gold this task,so unless you need the occasional clue or just want an easy task to kill time,these creatures are best blocked.
Cave Krakens

Cave Krakens,much like Hellhounds,have a high Slayer task weight,which means you'll be assigned these more often than you'd like. These monsters are particularly slow to kill since you can only use magic-based attacks,like a Trident of the Swamp,and you can't utilize melee or ranged combat.

This results in very low XP per hour. The Kraken's drop table also doesn't offer much in the way of profit,making this a task that's better blocked if you value efficiency. Their high task weight makes them a prime candidate for your Block List.
Black Dragons

Black Dragons are a bit of a hassle to deal with,mainly due to their high Hitpoints and the need to bring anti-dragonfire protection (either a shield or an antifire potion). If you want to kill them efficiently,you'll need a Dragon Hunter Lance,which is an expensive weapon,and its use is generally limited to Slayer. Duradel typically assigns only 10-20 Black Dragons per task,but this isn't ideal for XP gains. We're looking for tasks where we can kill larger numbers of monsters in the same location for maximum experience,so Black Dragons don't fit the bill.

Additionally,while Dragon Bones are valuable for Prayer XP,many other monsters drop more consistent and valuable loot,making them a better use of your time.
Greater and Black Demons

Greater Demons and Black Demons are high-weight Slayer tasks that require a lot of effort for minimal reward. These demons are single-target monsters,which means your kills will be slow,and their loot table is less than impressive. Duradel frequently assigns these monsters in large quantities,often around 130-200 per task.

The XP from these demons doesn't make the grind worth it,and if you're trying to maximize efficiency,these tasks should be blocked as well. For Ironman players,these demons might be useful for the Slayer Helm and other specific items,but even so,the slow kill rates make them better left off your task list.
Spiritual Creatures

Spiritual Creatures found in the God Wars Dungeon (GWD) are some of the most frustrating Slayer tasks in the game. These monsters don't have a particularly high task weight,but they are extremely tedious to cheap RS gold kill. They drop poor loot,and getting to the GWD in the first place is a hassle.