I thought it would be fun to post up some figures from the xbox side as well
The true achievements site is really useful as it lists stats based on their registered population AND it can now show the whole xbox population as well.
Taking Dark Souls, which was a "free" game with gold, the TA percentage for Bed of Chaos is lower than on PS at 23%...BUT the xbox wider population is even lower at 14%.
I'm not quoting these to refute the OP, but to show that "selection" of populations can have a big effect. e.g. moving from a general population to one keener on achievements (TA) almost doubles the percentage.
Assuming xbox gamers are similar to PS ones, then the game being "free" has dropped the percentage from 37% to 23% (assuming TA gamers are like PSNtrophies gamers).
Another way of trying to look at the drop off from "free" games is the difference in the very first achievement "enkindle". It's for lighting a bonfire, is unmissable, and is kind of the point of souls games! Almost 100% of TA gamers got this achievement but only 88% of the general xbox population. So 12% of people have loaded the game and not made it out of the first couple of rooms...possibly trying to take on the demon rather than run past it!
So how does this compare with other games? I've picked 3:-
Destiny - first achievement - ship rite - 100% of TA BUT 66% of Xbox!!
EDIT - scrub that - i forgot it doesn't have an achievement for actually completing the story/campaign! As an aside - completing a raid is at 20% for the xbox population but I think that is quite high considering you need 6 people, and they can take 2 hours to complete or more...plus only 66% made it to the first mission so technically a third of those that completed the first mission went on to complete a raid.
COD BLOPS 3 - I'm assuming the first achievement is "a second chance". TA is 44% BUT Xbox is only 8%!? I'm not sure what has gone on here...completing the campaign is also very low - 21% and 2% respectively!
I'll try COD IW - this looks more like it - the first achievement is at 83% for TA and 40% for Xbox..the complete campaign (operation bloodstorm?) is at 38% and 13%.
Trials Fusion - IMHO this game has a very good difficulty curve, but is obviously more niche. The first achievement is at 96% and 48% respectively. The last campaign achievement - all hard tracks completed - is at 39% and 10%.
Ok...so what does all this mean? How do we want to define "hard"? We could base it on the straight completion percentage? Or we could choose something else, like the drop out rate through the game.
The drop out rate for Trials fusion is pretty high, out of the 48% that made the first achivement, only a fifth of them made it to the last campaign achievement. In Dark Souls the completion percentage drops from 88% to 14% which means only one sixth of people made it through the game.
Looking at a mainstream game like COD IW, the drop out rate is much less...over one quarter of people made it through the campaign.
So in summary, IMHO, DS is not any harder than other "hard" games like Trials...but is harder than mainstream "easy" campaigns like COD. The raw trophy/achievement percentages can look quite high because of selection in the population (e.g. most people buying DS/BS etc do it knowing what they're getting in to). Off the top of my head I'd suggest looking at drop out rates through the game as a better metric because it helps eliminate some of the selection effects and things like games being GwG.
If I knew more about games I would happily compile a table of first and last achievement percentages, and drop out rates, but it's a pain looking each one up to check what the last campaign mission was.