Took me a long time because i had to restart from scratch at some point of my first run as i didn't had fun anymore due to the constant near impossible odds.
My mistake was my original choice (trying to keep Jonathan Reid as a good guy by murdering no NPC), this made my character simply too weak in comparison to how the difficulty progression had set my enemies (and me not really managing to get used to my keys choices, that was a disaster), and mixed with my a bit underestimated difficulty level this wasn't fun for me at some point.
On my 2nd run some NPC were turned into food to help Reid rising in power quickly.
... And what a difference it did as the only difficulty were then the bosses fights (and not all of them) as once i got used to my dodge key , along his "powers of massive killing" my character became nearly unstoppable doing an absolute carnage in the ranks of the skals, vampire hunters and other monsters that in my previous run were too strong, tanking everything the enemy was throwing at me with a smile (maybe he was just ready to drink more blood) instead of being nearly one shot everytime.
Interesting system to build your own difficulty and challenge, refrain to murder npc to keep your strength in check or go full serial killing monster and become unstoppable. Though it's a bit hard to judge where you should put the limit to not overtake the difficulty progression if you still want to be challenged a bit, as i went beyond that limit in my run (those poor NPC paid a big price
)
Anyways, i was really impressed by how very large and detailled the London environment was when free roaming around , either for some refreshing night tour, or just for grinding items and XP, especially as i was often walking instead of running (because i noticed the streaming engine sometime paused to dynamically load the environment if i was moving too fast, while walking everything was completely smooth and seamless) .
And also very impressed how the many NPC have their own voices, their own tones that fits so well their characters and the situation in the era the games takes place into, the amount of vocal dialogues is incredible and contribute to build up the atmosphere superbly.
Also each of those NPC have some kind of background story and mini storyline (sometime mixed with other NPC) that you can follow for a while by finding/getting hints with your actions in some part of the town, your discoveries and dialogue choices (assuming you don't outright murder them for XP) or to their end.
I felt the town management with the diseases and you manufacturing cures and travelling to give them to the NPC also took a far away background role the more overpowered i got, because you're so strong that you don't care anymore if a sector danger level rise or if the sector simply fall in ruin/ goes full hostile, as if you get too powerfull the stronger enemies can't do anything to stop you.
So i think that to keep that part of the gameplay relevant, you need to refrain from killing too many NPC, but that is also an interesting choice to do instead of the game forcing you to balance things, so if you think it's too hard you can overpower yourself (but unfortunately it's one way, you can't depower yourself a bit if you got too strong).
Anyways, the global overall story was very servicable even if a bit drown by the many, many personnal stories of the NPC.
Oh and for those that like the free roaming into that huge city, an important notice : once you're in chapter 6 there's a point of no return (once you reach the area that will have the boss of that chapter) that will close you to the free roaming in the city completely.
The next chapter that will end the game takes place in a different and much smaller location. And with no postgame (as the game just end when you reach the end of that chapter) you will not be able to go back to free roaming around unless you start a new game.
So if you want to keep free roaming after completing the game with your current character, make sure to make a backup of your save (as the game do not have manual saves and will automatically overwrite it with every action you do) to replay it after ending the game.
Got one of the bad ending with all the carnage i did with the NPC (but not really bad ending for the monstrous psychopath i had transformed Reid into, it's more a bad ending for London
)