I played Demon's Souls when it came out and put it down and never picked it back up after like 6 frustrating hours. I avoided the series until Bloodborne, which I bought, installed, and literally never played. I let a friend borrow my copy and recently grabbed Dark Souls III to start tinkering around with. If you go into DS3 with no prior experience, it is pretty ruthless. Even the tutorial boss was a pain in the ass for me. Then you get thrown into Lothric Tower and get ambushed by Dragon's. those annoying Pus of Man things, and the Lothric Knights that will turn 1 small mistake into a 3-4 hit combo every time.
I can't comment until I get Bloodborne back and actually start playing it, but DS3 is pretty damn rough if you are new to the series. The way enemies just chain together attacks makes Stamina Management a huge part of the game that takes entirely too long for people to realize, and a few of the mechanics aren't well explained--it never told me early on that having your shield up greatly decreased Stamina regen, and once I realized that some enemies became easily 3x easier.
All I can say is I got way more frustrated with Demon's Souls than I have with DS3. Demon's Souls seemed hell bent on just fucking with you. It was tons of traps and blind corners into enemies just for the sake of making sure you never felt safe and the entire journey felt like running an obstacle course more than anything. Oddly enough the friend of mine that borrowed BB only put like 8-10 hours into it, and hasn't played since. He's been meaning to get it back to me, saying he can't get the mechanics down and it just drives him nuts. He had played DS2 or DS3 and made it most of the way through the game, so I suspect for him BB was way harder, but I think it depends on peoples play styles and past experiences.