There have been many posts about eureka as its own experience, but I would like to take the time and look at eureka as a relic experience. 4.0 made the choice to make the relics bound to eureka, and this really feels like a step backwards from previous relics.
The role of the relic:
Relic weapons have always been a top tier item released significantly later than the savage weapon counterparts, as a slower, but easier alternative to the ultimate endgame gear. They fill the role of casual endgame, for people that play a lot, but don't necessarily want to do the hardest content (although they can always do both). To make up for this, they have their own story and are treated as special items, particularly unique in glamour and glow.
Weapon story:
We have drastically changed the story of the relics, in 2.0/3.0, the weapons themselves have a history, or a soul. In Eureka, the location has a story, and the weapons are an afterthought tonally. In some zones you literally cannot make progress on your weapon until you have progressed the zone itself (Pagos). In almost all Eureka zones, the most lore about the weapon is a single paragraph from Gerolt saying "sure I will upgrade that". Regardless about personal opinions on whether the Eureka story is good or bad, it is not a story about the weapon, and the weapon is not made the focus.
Gameplay of the grind:
Prior to Eureka, the relics are split into different sections per patch, where the relic can eventually be caught up to the current savage gear (which never happened in Stormblood due to release timings of Eureka). Each of these sections is a different type of grind. In ARR, there was primal fighting, fetch questing, crafting, dungeons, maps, fate grinding, light grinding, levequests, monster hunting, tomestone purchases. In HW, there was dungeons, 24mans, fates, primal fighting, weekly quests, crafting turn ins, maps, light grinding, tomestone purchases, GC purchases, levequests. In SB, every section is feels the same. Anemos was NM grinding. Pagos was NM grinding followed by light farming. Pyros was NM grinding + bunny fates, followed by light farming. There have been a total of 3 mechanics in the first 75% of the Eureka relic, and the are repeated with no variety of what can be done. On top of this, Eureka is an instance, which means none of the weapon grinding can be done at the same time as other tasks (such as running leves or killing monsters while waiting on your FC). This also completely disables the interest/viability of doing later relics, as Eureka will become less and less popular over time. In contrast, 2.0/3.0 used the relics to revitalize old content (crystal tower was basically dead before the existence of aether oil).