I'll provide an example where the horizontal variety can make a real impact which is often discounted.
Replacing targeted -na spells will a universal has a significant negative effect on what the player can prioritize, in certain situations. Any time you only have 1 debuff on you the effect is more or less the same, although MP cost, casting time and recast time can't be set individually, but I wouldn't say that this is a huge loss as long as the catch-all spell has values towards the low end.
However if some attack (Bad Breath type attacks) or combination of attacks (a chain of WSs by a mob, or actions by different mobs in a group) happen to inflict a bunch of debuffs at the same time, being unable to target what you want to remove means being at the mercy of whatever mechanic is used to select what is removed.
The devs could design a priority order (silence is removed first, then para, then slow, then dots in a certain order) but that order wouldn't necessarily be optimal in every situation. Removing silence first makes a lot of sense for casters in most cases but is obviously useless for melees, except everyone can cast some spells (maybe not MNK DRG WAR depending what is excluded then, but all classes). Then in some fights a dot might do insane damage and be absolute priority while in another it does very little, etc.
Alternately they could use FIFO or LIFO (First In First Out, Last In First Out), ie remove the oldest or newest debuff respectively. Could also be based on remaining duration. Works, but a lot of the time the order in which you remove them won't be the order you might have wished for.
In XI you basically had a combination of the 2 types for white magic debuff removal (a range of -na for common debuffs and Erase as a catch-all for the rest, with Esuna being added later as an AoE version that could remove multiple debuffs and remove almost any type when certain conditions were met) but then dancer only had a catch-all with a ridiculously long recast and holy crap did it suck, especially when dealing with paralysis on you.
I suppose if the catch-all is nearly free, instacast with no recast none of those things matter, and almost any implementation would be better than Healing Waltz so we aren't doomed to something that bad, but combining the actions can make a real difference in this case, and there are situations where other common combinations being separatable can come into play such as with buffs and monsters with buff steal abilities, monsters that change element during battle, etc.
I'm eager to see how the class reform will play out in game but I think having concerns about some real losses in flexibility and the fate of the elemental wheel system is very legitimate given what we currently know.