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.
In books 1 and 2 you have two fireballs as part of your magic, each of which work on a roll of 6 or better and do 75 points of damage each. The main bosses for both books have 150 LIFE POINTS, and there are very few occasions outside of these boss battles where you will actually need to use a fireball, so unless you're unlucky the bosses could end up like this.
The final boss in Legion of the Dead, the series' Grand Finale, is Merlin himself and the entire title Legion — and no fighting at all is involved, just one fairly easy-to-obtain item, and if you don't have it you don't even get killed, just teleported elsewhere.
Game Breaker: The Luckstone in Book 1, which lets you add or subtract 3 from every dice roll from then on. You can even keep it with you for the next two books. Once you have it, the books become a bit easy.
Seasonal Rot: Realm of Chaos and Tomb of Nightmares clearly suffered from serious proofreading problems, offering several pathways and items that don't do anything, ignoring previously established rules and contradicting themselves repeatedly. Legion of the Dead also has similar faults, but not as prevalent.