I've very often witnessed that certain people find certain bosses way harder than others. Some people think Midir is the hardest boss. Some think Midir is one of the easiest bosses. Same with Nameless King.
I want to address that a boss's difficulty can be judged in different ways, and that you should keep these criteria in mind before telling someone "you thought that boss is hard? he's the easiest boss ever git gud you scrub".
How hard is the boss for a first time player?
I personally think Nameless King is one of the easier bosses. I've beaten him dozens of times now and know his moves. They are actually pretty slow and not that hard to dodge.
BUT I will also admit that the first time I played the game, Nameless King was by far the boss that took me the most time before I finally beat him. The timing of his slow attacks feel weird the first few times you fight him and easily punishes panic rollers. The unfamiliarity of some of his attacks, such as the timing on his lightning strike attack, or the entire spear moveset in general also make him harder to beat on first try.
And I think Midir is the same as this. "Beast" bosses generally have this. Midir is a giant beast with very sporadic moves, and the first few times you fight him some players can find it extremely hard to predict what he's going to do. Against a sword wielder when he raises his right hand, he's usually going to swing from right to left. But when Midir rises up with both his hands, he might either slap you with his right hand, swipe you with his left hand, or slam you with both of his hands. Or do a plunge and try to bite you. Or start shooting fire.
But after you beat Midir several times and memorize his moveset, you will notice that all of attacks have giant windups and are pretty easy to recognize and dodge. This is why new players and noobs often call Midir one of the hardest bosses, while veterans call him extremely easy.
tl;dr Some bosses like Nameless King and Midir are extremely hard for new players or the first time you fight them, while for some people they will start to feel extremely easy after beating them multiple times.
Bosses that fit this criteria: Nameless King, Midir, Champion Gundyr, Dancer and Blackframe Friede.
How hard is the boss after beating him dozens of times?
There are also bosses that stay hard, even if you've beaten them 50 times already. Dancer of the Borreal Valley is the only boss in the game whose attacks do not follow the same rhythm as the rest of the bosses, which can sometimes make timing your rolls difficult. The Dancer requires concentration throughout the entire fight.
Dragonslayer Armour has some pretty fast attacks, and also very hard hitting attacks. Because of this he requires patience and deep concentration throughout the entire fight. He's not one of the hardest bosses, but even veterans can regularly die to him when not paying attention for a second.
On the other hand, the bosses that are very hard for beginners, such as Nameless King and Midir, can become extremely easy after you've beaten them dozens of times. Nameless King's and Midir's attacks are actually slow and predictable, and Champion Gundyr is easy once you've memorized all his moves and don't get taken by surprise anymore by his aggressiveness.
tl;dr True difficulty lies in a boss that stays hard even if you have beaten him 10 times and memorized his entire moveset.
Bosses that fit this criteria: Dragonslayer Armour, Dancer, Slave Knight Gael (huge health pool after all), Pontiff Sullyvahn, Twin Princes.
How hard is it to cheese the boss?
I think this is one of the most overlooked criteria while it should be one of the most important things. Sometimes I see people say things like:
"Oceiros isn't hard at all. Just make sure you always stay behind him so his instant-charge attack doesn't hit you."
"Pontiff Sullyvahn is easy. Just stay to his left side and you will mostly be out of reach of his sword."
"Dancer second phase is very easy. Just always stay behind her and most of her attacks won't even be able to hit you."
"Abyss Watchers is easy. Just keep running 24/7 until he does that one very specific move that lets you backstep him. Repeat."
"Aldrich is super easy. In the second phase just stay close to him and he will never do his arrow attack."
You have to realize that these things don't mean a boss is easy. It means it is easy to exploit the boss's behaviour. If you actually fought these four bosses fair instead of exploiting their AI they will be a LOT harder. Backstabbing Abyss Watchers? Piece of cake. Actually fighthing him toe to toe like a duel? Quite a bit harder.
Especially new players will not know about these tricks unless they watch Youtube videos or read wikia pages. So before you tell people that a boss they have trouble with is actually very easy and that they need to git gud, keep in mind that they might be trying to fight them the normal way, not the cheese way.
tl;dr Just because a boss is easy to cheese doesn't mean a boss is "easy" and does not give you the right to laugh at people who think that boss is hard. They might be trying to fight him the legit way.