I am not a high level player, but I find MKX easier to learn and play than MK9, Injstuce 1 & 2, and SFV. Haven't played enough of Tekken 7 to have an opinion, but I find MKX easier than TTT2, for sure.
For one, MKX's inputs are smoother and more responsive than MK9's and I find it less execution heavy as well. For example, Raiden's and Sub-Zero's combos were harder to execute, for me, in MK9 compared to MKX. As for Injustice 1 & 2, they are both back to block, so that automatically makes it harder for me, especially with things like teleport attacks, cross-over jumps, and especially ambiguous cross-over jump (which shouldn't exist in a back to block game).
SFV is also back to block, but over there that's not as much of an issue as it is in Injustice 1 & 2 because teleport attacks and cross-over jumps aren't really a thing, though they may exist in a limited fashion (not sure about this though). The real problem for me in SFV is all the quarter circle forward and backward, and especially the double QCF, inputs. Also, the lack of a run button. I love the run mechanic in MKX, it gives characters a lot more mobility and enhances their ability to apply pressure (this applies to Injustice 1 & 2 as well).
As for the 50/50s in MKX, I find them a well crafted gameplay mechanic, just my humble opinion. I always enjoy good 50/50s as they make for great mind games. The ones in MKX are good, but not overpowered.
I find Tekken games in general easier than MK9, Injustice 1 & 2, and SFV. They have really smooth and responsive controls and are not terribly execution heavy. The back to block there trips me up a lot though. There have been many a times when I've tried to block and I've back dashed instead, which usually resulted in me losing a lot of health or the entire round or match, or when I've tried to back dash and I've blocked instead.
And lastly, I find Soul Calibur games on par with MKX in terms of learning curve and difficulty of execution. SCIV had the ridiculous Just Frame moves that required insanely tight timing, so much so that I would venture to say that was the hardest thing I've ever had to do in any fighting game. The Run Cancels in MKX were not quite as difficult, though still pretty challenging. Other than that though, SCIV was quite reasonable as far as ease of execution. SCV I found easier than IV and at the same difficulty as MKX.
The challenge for a player in a fighting game should come from, among other tests of skill, being able to execute difficult moves accurately and in a timely manner, not from finding workarounds for imperfect controls. I think MKX got that balance right and because of that, it's a very enjoyable game to play.
