They get hate because it's lazy. BLU's over-powered spells are actually pretty low in number, aren't what people usually think of as iconic blue mage abilities (which as an aside: I think is a stupid notion, the appeal is using the abilities that monsters use, not using the same abilities another blue mage did a decade ago), and could be adjusted to fit with the rest of the cast anyway. I'm way too lazy to exhaustively list what I think is good from the spellbook so instead I'm going to just go off of the spells used in the usual DPS raiding spellsets.
Spells that don't break the game: Song of Torment, Bristle, Moon Flute*, Feather Rain, Shockstrike, Sonic Boom, Whistle, Angel Whisper, Surpanakha, J Kick, Triple Trident, Tingle, Cold Fog**, The Rose of Destruction, Matra Magic, Phantom Flurry, Nightbloom, Winged Reprobation, Sea Shanty***, Being Mortal, Breath of Magic and Mortal Flame****.
*Not actually particularly broken just restrictive and kind of boring. Even normal BLU is better off with it being adjusted IMO.
**Strong for BLU, but not actually that strong compared to other casters' movement options and ppgcd.
***only the extra weather effect warps this skill
****Easily calculated into the pps of the job, only an issue in Ultimates where they would need to be reapplied a number of times.
Spells that are just broken, no argument: Diamondback, and White Wind.
Broken but could just have less access and be balanced: Off-guard, Candy Cane, and Magic Hammer.
Trait-tier, but technically very strong: Aetherial Mimicry
Spells that are too situational: Revenge Blast, Wild Rage, Final Sting.
The biggest sore thumb is probably the variety of Death or %HP effects BLU has. These can just be removed lol people only use Ultravibration and Doom now anyway. Missile also taken in groups on occasion (for example t13).
Suddenly it seems very easy for there to be an unlimited version of BLU implemented. One that people already play and enjoy. We will soon have jobs that require you to have completed ~3 expansions and a base game of content to access, so I don't think filling out a spellbook is a big ask. Even if it was just swap the spells limited BLU uses for a new set you just unlock as you level and complete job quests. There are a great many monster and boss spells yet unused that could fill such a purpose.
tldr: people hate it because you only get to seriously play the job you like for something like a month maybe every 3 years, it's perpetually 10 levels behind, and, at least for BLU, there's an obvious solution.
