Fatal blow takes a lot of aspects from other games that had comeback mechanics, I think they said themselves that the main reason for them being the way they are is because they were rarely used in previous games.
Shouldn't be surprising though as you spend your entire bar for a ~33% attack that scaled like ass instead of using it on the very useful EX moves, breakers, clash, pushblock etc. I dont even remember any X-rays in MKX being used in competitive play aside from Kitana's as a very expensive mixup tool.
Their answer was making them related to damage you take, which is.. fine I guess, it's become fairly common. Its their first stab at the hail-mary comeback tool, SF4/SFV did it and Tekken did it so why not.
Fatal blows, while very similar to Tekken rage arts have a few differences that make them feel more "cheesy".
The main thing is what many have already said, that shit comes back if you miss which is mind boggling. What kind of shit is that? Just like all the other examples, you should get it once hit or miss, it's really not that complicated.
It has to come back for the characters that can cancel it? they're using the resource of the extension they shouldn't get it again fuck them. Sub can cancel his X-ray in MKX and you didn't see him keep his meter.
Imo fixing this alone would be enough.
The other big difference between Ultras/V-triggers/Rage arts and Fatal blows is the no big flash before startup, hell with some like Liu Kang's and Jax i dont even realize what hit me till a sec in the animation, shit is blindly fast and have mad range.
I'm not expecting them to add freeze frames like the other games but its definitely a factor in how good they are. I'm sure the thought crossed their minds coz injustice series has that for its supers but they decided against it.
Also, comeback mechanics changing up the matchup isnt really new, in many cases it is intentional. A character that was getting zoned out all game suddenly has a tool that can punish a sloppy projectile if they read it (Rufus Ultra 2 in SF4, Vega VT1 in SFV, Bane's super in i2), or getting an extra tool (extra anti air, command throw super, reversal of some sort, good whiff punisher etc).
In MK11 however these types of KBs are outliers, and imo are on the wrong characters.
Lastly, I agree with that they're way too long, previous NRS supers were fine coz you'd see them once every 40 games at best, but when you essentially see FBs twice every game (yours and the opponent's) coz they're really good and why wouldn't you use them? yeah they are way too godamn long.