These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: The flame parasites. Not only they have the ability throw huge fireballs towards you for massive damage, but they always catch fire after killed and explode after a few seconds. Depending of the battle, you may not be able to get far from the explosion, since you may still be fighting the remaining enemies.
Scrappy Weapon: the only reason for the Flame's existence was to light flammable plants, and even then, there are other weapons (like the grenade) that can light them on fire.
Potentially even more useless is the Claw, which despite being the only unlimited ammo weapon, never really gets used except for breaking rocks once you get better weapons. It too suffers in that its purpose gets taken by future weapons (again, the grenade.)
Not necessarily. The Flame weapon seems to me the best choice for a short-range combat during the first half of the game, and it's still quite useful during the second one. All other weapons either are designed to a better use in long-range combat (such as the Spike or the Spore AKA grenade, you REALLY don't want to throw a grenade to an enemy in front of you), or run out of ammo very fast (such as Necrocyte or Electricity), what means you'll have to switch to another weapon after a short time if the fight is too long.
That One Level: Level 10. You must reach the top of an island before it sinks, making it a Timed Mission. Besides the usual Zerg Rush of mooks, you have to pass through several locked doors, which only open using keys available in paths apart from the path to the top, and collect a pair of exploding spores in order to open the side path where one of the keys is. The worst part is, not only you have a limited time to reach the top, but a limited time to grab the items at the side paths. Waste even the slightest time in a point, and the game may become Unwinnable without you even noticing it, as the sea leel may have risen enough to sinks parts of the next side path or even the item you must grab.