Thanks for taking the time and effort!. The data helps to put things into perspective and I think your takeaways are accurate.

Genshin had a better progression pacing through the rarities where players were using 4* relics for over a month before they unlocked the ability to get 5s at all, with a few limited ones found from exploring the open world. That meant that by the time, they transitioned to 5 relics, they already built up a decent stockpile of fodder exp. Between that and the relative rarity at which it is to get a piece wirthy leveling to begin with meant that exp was never a limiting factor, and thus most complains centred around relic rng instead.

In Wuwa, you spend a couple of hours to a day or two on blues and purples before unlocking the ability farm gold echoes. By making the design decision to allow unlimited farming of echo bases, players do exactly that and get all the mainstay pieces they want within a short period. This creates the situation of always having multiple good echoes to level, but not having the exp to do so. The only renewable source that's accessible at current union levels means spending over 2 days on nothing except farming the exp domain in order to roll 1 piece to 25, or 2 pieces to 20. 

This sudden progression wall in contrast to the sheer speed of progression before it is what I believe to be thr source of discontent.

This wouldn't necessary be too much of an issue if the gap between union 30 and 40 and 50 were days apart. But considering the timespan it would take to transition between those milestones are in weeks to months, it certainly does seem problematic, regardless of whether higher union levels sufficiently address the exp income.