A great deal of gameplay elements in many video games are chance based, especially so in MMORPGs. Since those have their roots in Pen & Paper RPGs and board games more than skill based arcade games this isn’t actually a big surprise. Instead of dice being rolled the game’s random number generator (RNG) decides if you succeed or fail to, for example, hit your target or get your hands on a piece of desired loot.
Is this good or bad though? After thinking about it a lot and weighing my experiences over the years against each other I have to decidedly conclude: it depends. š
Many forms of randomness aren’t really noticable as such. That my attacks sometimes miss is a fact that I just acklowledge and that I work against by raising stats like accuracy. But the important bit is that missing an attack every now and then is almost never a big deal. It doesn’t hurt much, is what I’m saying.
My first experience with randomness that could actually hurt was in Star Wars Galaxies. When crafting I sometimes failed, losing all materials in the process. I didn’t know how my chances for success were, nor if I could have minimized the chance to fail somehow (it wasn’t even known for sure if using crafting stations and tools of higher quality had any effect back then). After a while I was convinced that I failed more often on critical combines consuming the most resources, resulting in me thinking “RNG hates me!” for the first time. It wouldn’t be the last.
I don’t know how many times during my first years playing Everquest II I hoped in vain for a specific drop that I really wanted, or worse, saw it drop only to have someone else win the roll. For a while I really hated the randomness of it all.
As time passed and the genre evolved to more accessibility and less overall difficulty, token systems came into play. Instead of specific items (often only usable by a specific class) mobs would drop a token that could then be exchanged for an item of choice. This circumvented the frustration of ‘again a whole dungeon run and not a single drop for my class’, but made competition even more fierce since most players had ‘Need’ on most drops regardless of class all of a sudden.
Even further went The Secret World’s approach. Instead of tokens dungeon bosses would drop a fixed amount of currency. Every endgame weapon, armor, enhancement and upgrade would be bought with that currency. The good thing about that was that every player always got the full amount of currency, so there was no competition and no envy between group members at all. At first I really liked this solution.
After a while I felt that something was missing though. There was no pleasure, no adrenaline rush and no associated story when getting a new item.

In contrast, I still remember under which exact circumstances I got most items for my Warlock in EQII’s Rise of Kunark expansion. The Tormented Bracelet of Doom, for example. This could only drop from a contested named mob in Kunzar Jungle, Doom. I would swing by his turf every time I started playing and look if he was there. He was pretty sturdy and hit very hard, fighting him solo was a tough challenge. On the day he finally dropped his bracelet the fight was particularly close, I beat him with just a sliver of health left myself. I was overjoyed to finally get it, so much so that I still remember now, about ten years later, that it gave +4% to crit chance and +17 spell damage at the time without looking it up.

This made me realize that when it comes to item acquisition I actually prefer the much more random approach of yore. For me token and currency systems took pretty much everything that’s fun about getting better gear away.
Then there’s gear upgrading as seen in ArcheAge and Black Desert Online, among others. This seems to be an Asian thing, basically not being able to use an item right after getting it because it has to be heavily upgraded first to become effective. Chance plays a huge role here too.
I passionately hated ArcheAges’s system, which involved so much RNG and so severe consequences when failing that it made the whole thing downright hostile.
The RNG ‘fun’ began well before the upgrading part. When crafting armor or weapons the outcome in terms of stats was completely random, and most versions couldn’t even be crafted further to higher tiers at all, forcing you to start over.
The real pain started once you had managed to get your desired base item though: the regrading system. This boosted the stats of an item while keeping basic properties intact. From the second tier onward (of which there were twelve!) there was a good chance to fail an attempt. From fifth tier there was an added chance to downgrade by one tier if you failed. From seventh tier you could downgrade two tiers with one fail. From eighth tier onward you could lose the whole item. With every tier upgrade an item gained exponentially more power than with the previous one, making top tier items godly powerful, so just not doing it wasn’t really an option. I quit the game mainly because of this crap.

Black Desert has a system that works similar in outline, but is much, much more forgiving. I’m actually having quite some fun with it, though one important reason for that is the fact that the game keeps handing out upgrade materials, silver and lots of other stuff for free on a daily basis. This lessens the feeling of loss immensely when failing a couple of times.
In conclusion, RNG elements in MMOs can be extremely frustrating to the point of ruining a game as a whole for me. When done right though they are not only ok, they can actually enrich the experience by giving achieved goals much more gravitas.
Your mileage may vary of course.