It was a data transfer/data storage issue that was simpler and easier to solve with class design changes than trying to mess with the game's spaghetti-coded infrastructure. Pruning extraneous non-combo DoTs such as Fracture, Touch of Death, Scourge, Mutilate and Phlebotomize reduced debuff/dot totals to reasonable levels while also freeing up button space.
HW was where it was at its worst with BRD & SUM dps being hamstrung in alliance raids, Diadem, World Boss FATEs and S-rank hunts. Groups with multiple DoT heavy jobs often had difficulty getting full credit even when they had been there from the start of the fight.God, I haven't had that issue in a long while (I think it's been since ARR Odin), but since I've always had skills that are dependant on dots being applied, I could always tell when they were on (aka, festers hitting for the right amount, thundercloud procs, etc). It's not ideal, but in content with that many people ya rarely have to be 100% optimal. But seriously, what content are you doing right now to have ~50 BRDs or SMNs in one place? I don't think I've seen even eureka get that populated. ._.
Its not really a problem anymore since the StB debuff and dot purge. Since most jobs no longer have as many dots and/or debuffs the problem isn't occurring with groups as small as 24 people nor groups that are approaching 70+. If jobs had maintained their HW level of debuffs and dots were would be hearing about this problem regularly happening with Formidable, Archeotania, and S-rank hunts