RNG isn’t a bad element on its own, the way it is used that is the real problem. I won’t get into the archstone discussion because i don’t know enough about it (nor want to search about it right now…).
The classic way to use RNG tied to rewards is direct drop, in this method you (often) can have the information about which item drops from which enemy/map at the specific percentage, if you want X you need to do Y for about Z times (without limited amount of tries depending on the game) - completely straight forward.
Vaivora drops almost work like that, but are semi locked behind a limited content (yes, with some options to bypass it) and have an obscure drop rate, and these are the aspects that make the RNG feel worse. You can’t have an idea of how likely is to get a Vaivora equipment over each try, nor if it interacts with things like looting chance, nor if the chance is increased whenever they add new Vaivora equipment (which requires dev/staff confirmation due the low numbers). Even if you do get a Vaivora you don’t have control over what you get, great if you get what you want of something that is valuable, terrible when you get stuff that is at the bottom end.
Of course, there is the option to buy them from a lucky player but you still need that lucky player to exist and be willing to sell it, which may just not happen because the structure isn’t reliable. In the end, the best ‘strategy’ is to keep spamming a boring content to get money for direct purchase and pray to get something meanwhile, and you can’t just ignore it cause the game “difficulty” is adjusted towards the high end of equipment progression.
Vaivora RNG fails because it is (indirectly) attached to a content that isn’t fun and is unreliable at all ways it can be.
The worst part about all of this is that MMOs aren’t about getting rewards, they’re about choosing how you engage with the game (at you own pace and will), yet IMC keeps doing things that force you onto a straight path to be able to have fun, as they abandoned everything that isn’t within it.