The design foundations of WoW can be summed up in: work to improve your character. You level him, you gear him up. Harder content, better gear. And so it goes. This character improvement hamster wheel is, together with the social aspect, the very pillar of this game. It's what makes it have such a replay value, such addictive fun.
You want your character to be stronger, there's a path to make him stronger and you go after it. It's a social game where competition is an important element, even more at the endgame for raiders and so on.
Legion got so many things right. They added a great system of max-level character progression with artifacts. They're emphasizing the social aspects of the game, making players go out, run dungeons together, use group finder. It's working wonders.
But they just HAD to add Legiondaries. They were warned: it looked bad. Legendaries dropping from anywhere? RNGfest? Why Blizaard? They still continued with it. Annnnd... it's the worst part of the game right now.
The thing is luck is a part of gearing, always has been. It shakes things up, that's fine. Loot tables are fine. Then they added warforged/titantforged. Ok, more rng. Then sockets. Alllriiiight, more variation. Maybe too much rng going on, but bearable.
But explain: how can a game based on progression and competition award insane power increases to characters that did almost nothing to earn it, and deny this same power to other characters that worked hard for weeks and months to earn this same power increase? This goes against everything this game is about.
Yes, we all heard tales of lfr heroes that can barely turn their characters or pass tank dps with a bis legendary. Of players with an astonishing number of days played at 110 religiously doing caches, raids, dungeons and still haven't acquired a single one. Yes, right now competitive pve is a mess, as players of the same class are divided in tiers. Those that were lucky to get the right legendaries are able to rank high, they're on another power level.
Reports are coming of massive amounts of salt, tears and rage being generated at homes where WoW is cultivated. It's a public health issue.
The thing about Legiondaries, this awful system, is that it doesn't know what it wants.
It wants to be a sweet surprise, something you can't work towards, you can't farm, but when you get it's a great feeling. That's what the devs say. But if it's to be so, a rare surprise that can happen anytime for any player, they shouldn't be competitive. They shouldn't be so powerful performance wise. Because if they are, they're going against all that character development is about.
They're not a sweet surprise, they're cause of constant anxiety, envy, frustration. And when they drop they might actually feel really bad, if you were unlucky to get a bad one. That's the opposite of what it aimed for! Gosh, getting a good one brings more relief than satisfaction.
If you want to go with "sweet surprise" make them have cool effects but not affect how well you perform with your class.
Now if you want to make them powerhouses, fit them in character progression. They have to be earned, worked for. Add luck as a proponent element if you will. But you need to select how you get it, either by effort or skill. No one wants the boredom of MoP and WoD legendaries-for-all. But this lottery is a mess.
One way or another, players should be able to work towards a legendary if they affect their performances.
Right now hardcore players that try to farm them get a cruel, bullshit treatment. Of course you can't actually farm them, just roll more low-chance dices. They have to do all chores (dailies, lockouts), and carry many less geared/skilled players in mythics+ for weeks and weeks to improve their chances. Not only there's rng to get a legendary, there's rng to not get an awful legendary. You can farm a lot and not get anything, or get something mediocre at best. And after weeks of work you might be arriving at raids without a good legendary, while your class peers have and outperform you by a large margin, and lfr heroes walk around with those sick legendaries you lack. Just because they were lucky and you weren't, it has nothing to do with your dedication or quality as a player.
As they don't know what they want this system to be, they aimed at two widely different things and failed at both. They're neither sweet surprises nor do they work as more endgame content for character progression as they're far too random.
Honestly i don't know if legendaries should aim for sweet surprises or powerhouses. I don't even know if the game needs them at all. But as it is, it's such a bad inconsistent design!