Builds:
I have a big issue with a few systems added after hitting lvl 80 but End game builds is my main gripe. Characters all need specific memories/mods(for my Warframe fellas) to function and because of the way the game is designed(which is very poorly in this regard) you need 50 different layers of upgrades in order to make your character viable in any stages at all in the late game(6500+ BP range), those layers are all huge time investments and most of the time provide niche upgrades that aren't worth the time invested. And to make matters worse a lot of them lock your build a certain way, not allowing you to experiment(not that there is much to experiment with anyways) and to make matters worse those are necessary to engage in certain parts of the game which hold resources or more layers to improve your characters. Resonances and Harmony are clear examples of this. Heck, memories being tied to specific grid locations and only being able to be used by one character at a time is already ridiculous as it forces you to spend hours more farming for stuff to upgrade memories into different characters and resonating and then hypertuning. It sucks, it locks you to a build basically forever if you don't want to waste your time investment and then if a leap is added which changes the way your character is built like Lucia Crimson Weave's leap which adds a USELESS damage type with a janky build-up that TANKS your damage values unless you straight up ignore the ability. All of the systems involving builds need a revision INCLUDING the way we get memories and upgrade resources because buying them off a shop is the least fun farming method I've ever experienced in my many years of gaming.
UI:
Not much needs to be said here, needing to go through 78 different screens to collect stuff is unreasonable and overwhelming and objectively bad design. Wanna know what else is bad design? Useless currency conversions. Just use the base currency to buy what you need... (Wafers)

🤣