Urriak Uruk
Gaming is fun, and fun is for everyone
My point is that if you have to face 5 fire giants or more at Level 12 then its a TPK. In a fortress of 105 fire giants
You're not supposed to fight them all at the same time. If you do, you die. It's not an easy module, and to challenge level 12 players there needs to be a lot of giants. In each encounter however, there are rarely any times with 5 giants, and when there are they are not as strong as normal Fire Giants (they have slightly lower stats and are called Fire Giant Servants).
I partially agree with you on this but I think you have it slightly back to front (at least based on my observations).
The CR system begins to break down at higher levels primarily because the monsters (especially those from the Monster Manual) do not match the statistics for monsters of their supposed Challenge Rating (as per Page 274 in the DMG).
Now yes more options, items and spells CAN result in overpowered Party's but if you don't get the monster basics right you just compound any potential problems.
Githyanki Knight ( CR 8 ): 91 hp and two attacks averaging 46 damage.
CR 8 (by the DMG): 176-190 hp and averaging 51-56 damage.
Fire Giant ( CR 9 ): 162 hp and two attacks averaging 56 damage.
CR 9 (by the DMG): 191-205 hp and averaging 57-62 damage.
So the Githyanki Knight has 49% of the suggested HP for CR 8 and 86% of the damage. Githyanki Knight at 67% overall effectiveness.
A Fire Giant has 88% of the suggested HP and deals 94% of the suggested damage. Fire Giant at 91% overall effectiveness.
Both have fractionally higher than average AC's so we can cut them a tiny bit of slack. But we can easily see which one is more formidable on a per CR basis.
The CR system isn't breaking down because of what the DMG says; there is a mismatch there because for some reason the DMG assumes when you make your own monsters, you give them loads of HP and they don't hit as hard (which is reversed for many MM monsters, which do a lot of damage but don't have much HP). This isn't tied to why the Easy/Medium/Hard/Deadly classifications just break down in higher levels of 5e. Regardless of the balance of HP/damage, at high levels PCs mop up Hard encounters easily and are only challenged by truly deadly challenges. I know this, because I've run high-level encounters; the designers of 5E clearly didn't test enough high-level games, or maybe there's been enough powercreep by now that the core rulebooks don't estimate it well.