I actual like the 300 Spartan look Fyreslayer have going, on, maybe pants and capes can improve them, but i think the core design of shirtless berzerker dwarfs has its appeal. when people are drawn to collect this army it that certain that fiery slayer aesthetics
like other have said they are definitely held back by lack of unit, lack of variety and being very samey,
Vulkites look like they where design to be rank and file infantry in WHF to Karak Kardrin before the end time happen, Hearthguards look to similar Vulkites, their plefera of foot heroes don't really stand out amongst the army either.
i also think they are not that fun to play as either, they don't really have any flashy mechanics or that many different ways to play them. Magnadroth have always been that 200pt range monster and have never risen to anything stronger then that