DS2/3 I assume you're talking about Dead Space? (There's a few DS# titles out there)
I'd disagree completely with DS3 being tougher and I'd compare DS2 about equally - the only difficulty to DS2's was the save limit, each individual sequence was pretty trivial but the save limitations kept you on your toes. Plus having full information on things like your HP and full accessibility to upgrades (which when you bought clip size upgrades gave you a refill on ammo mitigating the reduced drop rate issues [and keep in mind in my case, I'm talking about without the abusive weapon tricks - foam finger, etc - those made it even easier])
Don't get me wrong, I'd not say it was a 10 difficulty wise - nor would I say DS2 was but with how enjoyable and engrossing the story is plus the difficulty level is the key. (Plus the extra immersion factor of being HUD-less for most parts of play)
(For sake of rating actual difficulty - TLoU Grounded 8/10, DS2 Hardcore 7/10, DS3 Hardcore 6/10)
Although do keep in mind that TLoU rating includes coming back to it after a year or so off the game versus immediately after beating the lower difficulty for DS2/3. Perhaps if I didn't have a year or so of rust on me I'd rate it closer or maybe even slightly below. Regardless it's a good journey in a very enjoyable game that never really felt like a slog.
Yes the save issue was (for me at least) what made it such a pain in the ass. There were sections that required luck and less gaming skill and could lead to an instant death which was extremely irritating. DS2 (yes, Dead Space) and its 3 saves took me a bit to complete but I enjoyed it due to the strategic sense that came from "what area should I save?". However, the DS3 setting where even one death, no matter what happened, would revert to total game over was excruciating and took me a long time to complete which is rare for me once interested in a game or trophy.
For TLoU I found myself being very careful, using stealth constantly, avoiding encounters I didn't need to engage in (such as the University Bloater & the Boiler room Bloater) and could waste ammo and supplies which I wanted to stockpile for the final few chapters. I started to realize that the runner packs were best to pick off one at a time by running away and dividing them. I also used a ton of bricks and bottles as weapons during my grounded playthrough (grounded + is easier by the way).
TLoU is easily one of the best games & stories I have ever played so it was a huge enjoyment to be able to get a new play mode. I think the most difficult encounters were:
1) Museum Runner Pack: This was my first introduction to how deadly they would be on that mode and they kicked the shit out of me several times before I changed strategies to dividing and bottle necking them.
2) Escaping Boston to the subway. I had difficulty with the final section with the 8 military guards in such a small area. I died here a few times having to restart from the area before it.
3) High School Runner Packs: The area before you entered the school. 12 runners is a lot and one group had 8 at a time which led to a ton of running. On top of that, there were no bottles or bricks in this area so that made it a hand to hand encounter as I wanted to save my ammo for the bloater.
4) The AI (to me at least) seemed much more aggressive and intelligent on grounded so I was often having to switch cover and use the throwing items very often to trick the pursuers. I had not a single issue in the Pittsburgh sections other than the initial crash portion but I, literally, watched myself move to different areas to set up better stealth angles against infected only to watch one immediately change its path route and disrupt my set up.