No, it's not. Numbers and the manipulation there of is a very parochial mechanic to use and results in a narrow style of play. Numbers will result in players being forced down certain roads because it's the path of least resistance, changing mechanics and intelligence allows characters to play how they want..but against smarter opponents.
Making ridiculously high saves for enemies forces players to methodically use spells like Divine Light, Drain Willpower etc and it doesn't make it hard, it makes it tedious. It's like having a huge equation with multiple variables, but the variables have been already figured out so all you have to do is plug them in..its not hard, it just takes a long time.
Basically, by changing numbers all you do is recreate the same hurdle to jump through over and over. Damage output vs damage received and using debuffs to saving throw abilities. By changing how enemies react and think you force new strategies on the player.
I love Dark Souls, one of my favorite series and I have played it since Demons Souls. One of the reasons people love the difficulty of Dark Souls series is that most of the time the added difficulty comes in the form of enemies having new abilities, changing their attack routines or adding another monster that isn't hard to kill but substantially changes the fight because of some small thing they do. Yeah, they have more HP, Res, Def and hit harder as well but the player isn't hamstrung and the numbers on the Evil NPC stat sheet are not the only thing that changes.