Having tried Eureka, I think that the majority of the reason people dislike it is due to a difference in playstyle and expectations. Mix this with what is, arguably to some, playing the content in the wrong way [zerging everything instead of forming a 4-8 person party, finding an empty camp with elementally appropriate mobs that are 6 levels above everyone's level and chain pulling them], and you'll have discontent with the extremely low xp per mob (because of zerg rushing the 'Easy Prey' mobs) and/or killing mobs not 'red' to you.
This zone really is much like FFXI - I played that game through Abyssea (stopped when cap was 90 with a WHM and NIN with mostly full AF3 and all the relevant atma). People need to approach it with a mindset of building a party, finding a camp, searching for appropriate elemental and level mobs, and grinding away while chaining.
This will be boring to many people, however, who have grown used to the way FF14 is structured and how questing/leveling/acquiring items has been thus far. It is a 'step back', in a sense, and people who have never played FFXI or a game like it [grinding the same mobs over and over just for minimal xp gain] will hate it.
I remember it taking as much xp to get from level 67 to level 75 in FFXI being about the same as it took to get from 1-67 // getting a level there often took a very long time, sometimes weeks depending on deaths, de-levels, and how often you could play and/or get a party (looking at you, dps :P ).
Eureka was just released, and it seems to cater to a play style that is foreign to many FF14 players, and is also one of the reasons some FFXI players left that game. However, I think if people can adjust their expectations, maybe adjust their methodology [mentioned above], and maybe have patience, it may not be so bad. No one is level 20 yet, and the 4.25 patch notes even say the best rewards/etc. come after level 20.
I don't think anyone is doing Eureka to increase their ilvl or for actual, usable gear. I think perhaps the main motivator is the relic glamour // in which case, the ilvl of the gear and the fact it will be (or is, in many cases) outdated shouldn't affect the grind, since the grind is for glamour and/or 'prestige' of having it once it can be completed. Just a few thoughts from a long time FFXI player who finds Eureka a fun experience if I approach it much as I did when making parties in the Dunes, Garlaige, Crawler's Nest, Cape Terrigan, etc..