Easy explanation for both, really.
Dishonored 1 is a power fantasy about being a supernatural assassin. It wasn't meant to be that difficult, it was meant for you to easily play and have fun. You will never wonder "am I good enough to kill that person?" because that will always happen but instead you're supposed to ask "how am I gonna kill you?" and have fun with it.
It's also not strictly about the missions and the targets but also about the world and the story. A good chunk of the game is spent approaching your target's location and this gives you plenty of opportunities to see what the world looks like, to meet the people that live in it and what they think, to get the story through somewhat indirect methods. This is why you never start exactly near your target, because Dishonored was as much about the journey as it was about the target.
This isn't to say it doesn't have problems but they all boil down to the same thing: lack of chalenging oposition. The moment you reach the Whalers should be the beginning of the ramp up, where every stage sees harder, stronger and more complex enemies, but instead you go back to regular guards instead. Daud's campaign solves this and is great for that.
Now, the hate stems from an overflowing amount of cretins. First you have the cretins at Bethesda\Zenimax that advertized the game badly. It was never a stealth game in the same vein that Thief was at all, it was never supposed to be "super hardcore", it was just a power fantasy.
Dishonored is closer to Prototype than Thief for instance.
But then you have the cretins that went to play this expecting Thief, either because they are that stupid that, seeing the magical powers given to you and the multiple videos about Corvo slaughtering people around, still expected a Thief-like experience or worse: they actually believed the marketing of Zenimax\Bethesda.
These are the cretins that can't understand the niche of "predatory stealth", wish that every stealth game play the same catering to them and they will buy games they should know they aren't gonna like just to complain about it later.
Then you also have the massive amount of cretins that plays videogames like it's a dayjob for them, trying to finish them as fast as possible. They'll complain about how short it is, how most content is meaningless and how a lot of things are redundant and you "gotta self-impose chalenges" because they can't just Blink on top of a rooftop and take in the view of a decrepit city being eaten alive by a rat plague. Nothing you can do for them, but just wait until they find out they aren't paid to play videogames…
Dishonored 2 likely suffers from all that but there's also 2 more issues.
First one is Denuvo and I'm not talking about performance issues. You place Denuvo on your game, you make sure nobody will talk about it unless they paid money, which means the usual people that pirate and then talk about it don't even mention it and there's nobody to talk about it with or dismiss any wrong information with it. No discussion, the game dies.
Then there's the actual performance. 50GB in your harddrive? Fuck that, that's a red flag for unoptimized crap nobody is willing to put up with. That's why so many people still haven't tried Doom despite being able to pirate it.
I personnaly saw it for sale at 20€ and was inclined to buy it until I saw that disk requirement, so that shit even wards off legit customers.