Ok. So, fascinating topic.
Ill start by giving my opinion.. and that is; No. MKX is not the hardest fighting game out there in any catagory I can think of.
Now, before we start to set things out, I think we need to define just what "hard" actually means in this instance, as it could mean different things to different people. I *do not* factor success into the definition of a game's difficulty. ie, if you were top 10 ranked in the world in game X, and after the same effort invested are totally unable to make it into that same top 10 in game Y, it doesnt mean game Y is harder than X. Success is different than difficulty in this instance.
I define a fighting game's difficulty as a combination of all the skills required to play the game at an above average level, and the investment in total effort required to obtain these skills - spread across an average of the game's characters. Ie, we arent cherry picking the easiest or hardest character to make our point, but rather trying to find an average between the two that the majority of the playable cast occupies. I assume the player has an understanding of FGs as a whole and understands the acronyms, terms, what frame data means, and the like.
In FGs that general breaks down to something like:
-General game knowledge. - This is understanding the universal systems in the game, as well as nuances that are required to know to perform at the above-average level mentioned.
-Character specific knowledge. - This is knowing your own character(s), and knowing enough MU related knowledge about other characters to play the CHARACTER on as even a footing as the MU itself allows.
-Execution required at a practical level. - This can be movement, BnBs, punishes, MU specific stuff that may be required to play vs a certain character correctly, etc. Practical, required execution. If a character has a combo with four 1frame links in it and that combo is REQUIRED to play that character correctly, then it counts. If its just a fringe-scenario/swag/justcauseican combo, then it does not. Anything that requires mechanical/physical skill.
Of course, this is all still quite relative. Difficulty in anything is heavily individualized. What is enormously difficult for one person may be absolutely basic and simple for another. A person may take to one game easily, and another only after many long and grueling hours of practice and effort.. While perhaps completely failing to ever acclimitize to another.
To the meat of the matter.
MKX is not anywhere near as hard (IMO) as numerous other FGs. It's not easy - I am absolutely not claiming that - but its over-all difficulty is not particularly high.
I'll state three primary examples that I think are a hell of a lot harder than MKX.
The first is Tekken 7.
So, the Korean Back Dash Cancel is probably the first thing people think about, that and Electrics. Tekken is sort of legendary for its difficulty and I think it lives up to that - but not because of the KBDC or EWGF, though they factor in. My reasons are as follows:
Movement. -
The KBDC's difficulty is *WAY* over-hyped imo. People talk about hundreds of hours and months and even years of training and all this nonesense.. and no. Unless on some personal level you just struggle with it. Using myself as an example, I have very weak execution across the board.. and I had a useable one in about 10 days, of sub-20m practice sessions and in a month it was excellent. This was on pad - but I practiced three times on stick for bout 20m each time and was well on my way to getting it there too. It's not easy, and its absolutely REQUIRED - so it does count in our discussion - but it's not anything like it's made out to be.
Tekken's movement is an entire world unto itself, and movement as a whole is what really takes months to years to learn in Tekken. It's not just the mechanical requirements of the KBDC or the wavedash (not everyone needs to WD but there are enough characters that do that it's relevant), but the knowledge. Understanding when to side walk, or side step, which direction (it's not one sidestep fits all), when to backdash, how far, how to use your KBDC intelligently and not just to scoot backwards quickly, various types of command movement(s) on different characters, and how all of these options funnel in to defensive and offensive situations and the various mindgames around creating whiff punish scenarios. In Tekken just hitting Down is a movement option that changes and significantly alters the entire movement "landscape". It's an absolutely enormous amount of crap to juggle and keep straight in your head and it absolutely *is* required to play the game at an above-average level. Movement in Tekken even plays heavily in to frame data and such, as some transitions change a move's OH/OB properties (this is arguably a MU knowledge kind of thing), and even when negative - depending on how much - side stepping can allow you to continue offense or entirely change post block situations merely by being an option that exists.
MU/Character knowledge. -
So, ya know how, in some games there is a character that may not be that great, but if you dont know his/her specifics, that character will absolutely annihilate you? Take, say, Bane in Inj2. He's not OP. He's not top 5.. but if you play him and dont know the MU and understand HOW to play against him, he can feel utterly and absolutely BROKEN, when played by a competent player. Blanka and THawk in SFIV were like this as well.. Well, in Tekken, that's honestly 75% of the cast. In Tekken MU knowledge takes on a life of it's own. Characters have upwards of 70 moves in most cases, pushing in to the 80s and 90s and even 100s for some of the cast. MANY characters have weird strings or moves that arent actually any good at all.. *IF* you know how to counter/deal with them. Lack this knowledge though and you can be utterly run over by gimmicks and dumb shit that seems broken or impossible to deal with. Hwo has a move thats multi-hit armored, leads to a combo that takes 50% of your life, is not just SAFE but on block acts as a small GUARD BREAK guarenteeing a small damage combo, tracks (cant SS), is unreactable on startup and can SET UP A 50/50, where it's other option leads to just as much damage... AND if you somehow catch it early enough before it's armor kicks in - the Hwo player is in a float state, which means you do extremely reduced damage... Understanding when a character can use stance or movement transitions to effect what happens after certain moves, etc. Then roll the movement options back in to all of this.
Damage. -
MKX hits hard. Combos do a lot and its unforgiving.. but Tekken is more-so. Matches play out over three rounds, not two, for a very good reason. Watch Tekken matches, look at the number of perfects.. The game is BRUTALLY unforgiving with most characters doing 40% or so off almost any launch, and mini-combos making 2 hit combos take 25+%. Throws in Tekken do a ton of damage as well. Get your back to a wall and try to deal with everything that's coming.. Its a nightmare. Many combos can convert if they reach a wall, and do absolutely brutal damage. You cant block a mid while crouching in Tekken, making the ability to "50/50" in Tekken not only universal, but absolutely constant, with incredible safety to the person making you guess. MKX is brutal when you're under pressure, but nothing is quite like being under pressure with your back to the wall in Tekken.. ESPECIALLY if you just got knocked down and your trying to get up.
Execution. -
I list this because I dont think, IN AVERAGE TERMS, execution is harder in Tekken. Yes, if you play a Mishima, or Lee or something, then I think that exceeds anything MKX has to offer, but in terms of average character difficulty, I dont see either game being above the other. Most character's BnBs are easy, but both games have some characters that are harder than others.
**other game comparisons to follow**
