Well, what one person finds unreasonable and another finds reasonable is subjective, but there is a mechanical way to look at difficulty.
There are skills, bugs, cheating (specifically in the case of AI), and punishment. Then there's RNG, which is a wild card. Game designers may snicker at that remark.
A skill-based difficulty is when you are attempting to surpass a barrier via a correct series of button inputs, positioning, strategic prowess, or otherwise, which rely on you sufficiently understanding the mechanics of the game to use them. A good example of skill-based difficulty can be found in Ori and the Blind Forest, and Undertale--both have clearly defined, well designed mechanics, that require you to use your natural aptitudes to surpass the obstacles in those games. In the case of the former, it's usually more about the overall sense of where you are and planning your next move out a couple seconds before you do it, thus chaining actions to go as fast and acrobatically as you can across the terrain. In the case of the latter, it's all about pattern-recognition and reflexes to learn about and keep up with the bosses.
Bugs are barriers of skill that are usually unintended and are either pernicious failures of game design at a fundamental level, or inconsistent programming errors that pop up sometimes. Hitbox clipping is an example of a bug, where two hit boxes can collide, but there is a failure to register it as such, allowing two models to flow through each other without any effect taking place. Another type of error can be based simply on a mathematical calculation error, causing an ability or skill to scale improperly, or to have base values higher than what was intended. League of Legends is overflowing with bugs of both of these types every time a new major balance overhaul comes out.
Cheating for AI is an often times necessary difficulty enhancement to allow an AI to function in the first place. For example: In Blizzard's RTS series "Starcraft" the AI has no fog of war. Lower difficulty AI's in Starcraft 2 behave as though there is one, but on higher difficulty settings, take advantage of the fact that they can see things you can't. Other examples of cheating include giving the AI skills the player never has access to, allowing it to perform tasks that no player could feasibly perform, or otherwise intentionally shifting the balance of power in favour of the AI. When done well, the player never notices and simply thinks the AI is smarter than it really is. When done poorly, it's so blatant and infuriating that a player will often cease playing the game out of frustration that the game is rigged. Much like the Democratic Primaries.
Punishments are points in a game where something damages your ability to progress or sets back your progress in a manner which could not be predicted, and which exists solely to stretch out game time, or to make the game appear more cleverly designed than it really is. However, not all punishing game design is necessarily bad. Some games (like Dark Souls) exist specifically to capitalize on this, to punish your ignorance and force you to learn and adapt and fail several times before finding a successful strategy. The original X-COM titles also have plenty of punishing game design: Unless you've played before, you'll have no idea how horribly dangerous chryssalids are and your first encounter with them will probably wipe out your entire squad and possibly GG the entire game. Night terror missions on your first run sound far less terrifying than they really are when the aliens can see much further than you can (cheating) and obliterate your soldiers as they merely step out of the Skyranger. Cleverly designed punishments usually explain some harsh element of the world, or service to set back your progress in a story-perspective. And, well, again, some people just enjoy smashing their heads against a brick wall until they break through it by sheer trial and error. (I beat X-COM 1 & Apocalypse on Superhuman each one time. Never again.)
RNG (or for those who are marvelously acquainted with the glorious bastard that he is, RNGesus), are randomized elements of the game you have no control over. A basic example of RNG is if a weapon swing in an RPG does between 6-16 damage: You have a random number generator (RNG) pick a number between 0-10 to add to your base value of 6 to determine your damage output. Critical strikes, failure chances, and so on, are all examples of RNG at play. Entire games are built around RNG (like Darkest Dungeon), and it usually services to provide a random element that makes conflicts more exciting. On the other hand, if the RNG is too stronk and wild, then it makes planning anything at all completely impossible. Good RNG forces players to think more conservatively and plan for failure, and sometimes force them to live with it. Bad RNG acts as just another punishment. The more RNG there is in a game, the harder it is to balance too--a lot of RPG's suffer from some weapon types or spell types just being blatantly better because RNGesus is more merciful to them, or end up feeling bland because there's no difference between most weapons.
Combinations of the above five form the mechanical sense of difficulty. Depending on what a player is looking for, and what their definition of reasonable is, some games will be more or less enjoyable for them. For example: I love strategy games. I have a blatant preference for science fiction in the stories I consume. So, by no great surprise, X-COM is one of my all-time favourite franchises, in spite of the bugs that plague later entries, and the punishing difficulty that forces you to deal with loss and failure on a regular basis. I'm fairly lukewarm to most action RPG's--I typically get bored of them--so by no great surprise, I don't enjoy Dark Souls all that much. I more readily see the errors and flaws in its design and they bother me more because I'm less enamored by the entire genre to begin with.
Plus, in spite of horrible design, some games can shoot up in popularity anyway by virtue of other factors. Skyrim's difficulty for example is the most artificial garbage ever envisioned by the studio. There is no sense of progression, hitboxes are buggier than in any title they've released post-Morrowind, the AI is so amazingly stupid that in one patch dragons flew ass backwards to the edge of loaded cells and then got stuck in the sky, magic is fucking broken in so many different ways... Yet the game gives you a lot of freedom to play with. It gives you a massive, gorgeous world on the eyes, and the modding community works night and day to add tens of thousands of pieces of content to that game. I have played 225 hours of Skyrim according to Steam, mostly because the world is pretty, and with mods, I can get lost in it for days at a time. People definitely don't buy Dead or Alive Beach Volleyball titles for their, erm... Realistic physics or jaw dropping gameplay. Mortal Kombat is most definitely not the most technically competent fighter game that's out there, but it's one of the most popular. For bloody reasons.
Hope that's helpful to you or anyone else that's curious about difficulty from a mechanical point of view.
tl;dr: Difficulty is comprised of a few aspects, all of which can be measured and done well or poorly, but whose interpretation of "reasonably crafted" differs from individual to individual.
