They made Zidanes trance/ dyne attacks useless compared to the other FF main characters
FF7- omnislash, much better than grand lethal
FF8- renzokuken the lion heart, like 30 times more better than grand lethal
FF10- Blitz ace, not that good but still beats grand lethal by miles,
What o want to know is, why on Gaia is Dyne so weak, yoou can eventually get to the point where his basic attack does as much damage as his trance.
What do you think?

Bosses have a lot less HP in FFIX, especially at the end of the game. I think they might have been trying to correct the problem of having an ultimate attack that does 20 times as much damage as the next best, like Omnislash and KOTR in FFVII and Lion Heart in FFVIII.

What makes Grand Lethal especially unimpressive, though, is the fact that, so long as you know what you're doing, you can make Frog Drop and Dragon's Crest, which cost 10 MP and 5 MP, respectively, and which can be used at any time without a trance, do 9999 damage, equaling the power of Grand Lethal. Actually, those attacks are better, because the status effect Mini won't affect them, and they work on physically immune monsters as well.

As for FFX, the ultimate attacks in that game are very broken once you break the damage limit.

A further point/tip to add is that with a decent Str stat, it is a little more MP efficient to use the likes of Scoop Art and Dyne instead of Grand Lethal whilst inflicting the same damage.