It's awesome, powerful, unstoppable... but not as useful as it might seem (if it's useful at all).
Yes, it seems that the designers put so much time into maxing out the "ultimate" factor of the ultimate attack that they forgot to actually make it usable. Maybe it requires too many resources to use, causing its allure of "awesome" to be lost as fast as your party's money. Maybe it requires some sort of bizarre set-up to enact, making your normal attacks and spells much easier to apply inside of battle. Maybe it has a considerable chance of failing or backfiring that makes it unreliable from the start. Or maybe in mathematical terms it doesn't deliver as much bang for the buck as "inferior" alternatives.
Whatever the reason, it will get used once, as a test drive, and then never again. Yeah, it's amazing, but you've got a game to win here. It's Awesome, But Impractical.
Keep in mind, Awesome, But Impractical is not Cool, but Inefficient, which is where something appears to be awesome, but has no real benefit to using. Awesome, But Impractical is genuinely useful, it just requires such a cost that you're probably not going to use it very often. Sometimes, Awesome, But Impractical moves carry a situational advantage, or are so Difficult, but Awesome that even highly skilled players have trouble using them but there is an advantage over normal moves if you can afford to pay the penalties. Sure, maybe that mini-nuke will irradiate and possibly kill your party, but if you're in a big enough pinch, possibility of death beats guaranteed death, so you may as well try it.
The very act of using a million tons of firepower on a few weaklings (a.k.a overkill) is also Awesome, But Impractical.
Related to the Bragging Rights Reward and Inventional Wisdom on occasion. See also Useless Useful Spell, Blessed With Suck. Contrast Too Awesome to Use, Boring, but Practical, Game-Breaker, and Simple, yet Awesome. Compare and contrast Difficult, but Awesome; there, the focus usually is on Impractical turning out to be Awesome. Crosses with Death-or-Glory Attack when a miss will result in nasty consequences, and Powerful, but Inaccurate when lack of accuracy is the reason for the impracticality. Scary Impractical Armor is a Sub-Trope, as well as Impractically Fancy Outfit. In Real Life, this trope is often the reason behind I Want My Jet Pack, and why a Rare Gun is rare in real life but common in fiction.
Please add only examples whose impracticality is shown within the work. If it works fine in-universe but later thought renders it clearly impractical, that's Fridge Logic.
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- In Motu Patlu, a lot of Dr. Jhatka's inventions sound cool or even awesome on paper, but in practice they get people in trouble more often than not.
- Discussed beautifully in Vision of Escaflowne Abridged.
Merle: Fossilized dragon hearts, stupid. They use them to power Guymelefs.
Van: That's why I had to kill a dragon before I could claim Escaflowne and become king. In a war, the side with the most energists usually wins.
Hitomi: ... Huh. And Zaibach mines them in mass quantities?
Hitomi: And your people get them by killing live dragons in single combat?
Van: Yup! ... When you put it that way, it sounds pretty stupid, huh?
Hitomi: A little bit, yeah.