Honestly I'm conflicted. The game feels good in a some ways and bad in others. I want to preface this by saying that I'm a fan of your work, so any criticisms are not meant with any kind of cruelty, and come purely from a place of seeing the amazing potential your games have and wanting them to be the best they can be, so I hope I don't come across as mean or harsh.
The playable characters are where the game really shines. You've done a great job of getting all of the classic moves, plus the fatalities/brutalities/friendships into the game, and I think this might be the most balanced your characters have ever felt, compared to Infinity War and Justice League where some characters just felt obviously weak or strong. For the first time I feel like I could select any character on the roster and feel at home with them

I haven't had a chance to play with everyone yet but the characters I have played as all felt completely viable, and nobody has felt overpowered or weak, they felt just right for the most part.
I do have a little suggestion for Jade though. Her fatality simply being two staff jabs is a little underwhelming, considering she has some really well-known ones that would've made much better choices, such as the impaling fatality (where she kicks them into the air and then they land on the staff) and the one where she jabs the staff into an enemy and then shakes it violently until they explode. I don't know for sure but I would imagine either of those wouldn't be too difficult to make?
It was interesting to see that projectiles and specials cost no MP at all and it made a really nice change to be able to feel completely free to use them as much as I liked, and the game is designed in a way where it's not overpowered either. It's a unique choice and one that definitely fits a game based on fighting game franchises (since you don't have MP in those games). Sometimes it's a bit awkward trying to use the Up, Up and Down, Down inputs for the Special and Uppercut moves but it's not too bad and will probably get easier with practice.
The stages are so-so. They serve their purpose but they often felt kind of bland and repetitive but I understand you're using Mortal Kombat assets for them, which were of course just single screen fighting arenas originally, so it's to be expected they wouldn't look quite as interesting in a side-scrolling setting. I did like the usage of some of them and the nice touches like being able to uppercut enemies off of the bridge or into the acid pool
The game's biggest flaw, however, is the same as your other games and the reason I won't be playing very regularly, and that's the enemies and bosses. For starters there are too many enemies (especially for one player to handle by themselves) in a lot of areas. Enemies appear to have ZERO delay OR recovery on attacks. Their attacks come out seemingly instantly, giving you no time to prepare or dodge or use any kind of reflex, and once they have performed their attack, they can IMMEDIATELY perform it again, because there's no recovery afterwards. So enemies that have a fast knockdown attack can repeatedly keep you on the ground because the moment you stand back up, they just hit you back down again instantly, especially in a crowd of enemies where if one enemy doesn't get you, another surely will. And of course there's no breakout move, so you have zero defence when surrounded.
The block function is almost completely useless. Once you get hit by anything, it BREAKS the block, and with so many enemies on screen, all that happens is you block one hit and then immediately get hit by something else 'cause odds are at least 3 enemies are all swinging at you at the same time, so you block one of them and the milisecond your block animation ends, one of the other 2 hits you. Having the reflexes to block should feel rewarding but instead it feels punishing.
The first stage is, ironically, one of the most annoying. The enemies with sticks are a core example of what I was just talking about, they attack way too fast, and of course they have the range to boot. Getting one of these guys on either side of you is miserable and you suddenly feel like a pinball stuck between two flippers ;D The ninjas that perform jump kicks are another good example of this. And again in both cases, if you try to block another kick or stick just hits you instantly.
Then of course there's the bosses like Goro and Kintaro, who use the same formula as so many of your other bosses from previous games. They tank everything, have a super fast punch that sends you flying across the screen, again with no delay or recovery, so simply trying to get to them and hit them becomes a chore, and the fights just feel really boring as a result; walk up to a boss, punch them a couple times (feeling like you're doing nothing because they don't even react to it), get backhanded across the stage, rinse and repeat. It becomes really tedious really fast and feels more like a test of patience than a test of skill. Even the giant machine gun demon is guilty of this; only he has a rapid firing gun (also with almost no delay between shots...) on top of that.
Bosses with projectiles (Shao Kahn and Kano in particular come to mind but I'm sure there's others) seem to have gone to the Ryu School of Hadouken Spamming, because once you're at range, they will shoot, shoot, and shoot. Even on the 2D stages where you can duck, you don't have time to duck, stand up, move forward a little, and duck again, because they shoot so frequently that there's just no room to move without getting hit. So your only option seems to be to keep trying to jump in and hope you make it eventually.
There's a fine line between 'challenge' and 'cheap' which, without trying to sound mean, your enemies and bosses tend to lean towards 'cheap' and it creates a game experience that isn't fun and just feels frustrating to play.
One thing that perhaps made this game more bearable was the length. Your previous games main story modes have been very long (roughly 2 to 2 and a half hours to complete, or even 4-5+ hours if you play all chapters in Justice League in one sitting). So whilst this game was frustrating, it at least didn't keep me frustrated for 2-5 hours, lol, though I'm not sure feeling glad the game is short is necessarily a good thing ^_^"
The sign of a great beat 'em up is when you complete one playthrough and immediately start another one because you're in the zone and loving the feel of the game (which is how the SoR franchise always makes me feel). With each of your games, I complete a playthrough and just feel exhausted and downtrodden and can't even appreciate the victory 'cause I'm not sure it was worth the stress of getting there. Then I quit and maybe don't play again for several weeks. I can see this game being the same kinda deal, though I do intend to play quite a bit more in these early days until I've tried all the characters at least.
I also know that you're very well-known for lots of post-game support and bonus content, so I'll be keeping my eye on future updates to see how the game grows from here

I think you have a good foundation. In it's current state I'd give the game maybe 6/10. I love the complete roster and character design, but the stages feel a little boring (again though, I can understand this to an extent) and the enemies just completely ruin the experience for me.
Anyway I don't wanna ramble for days but hopefully this will help in some way. I also hope I'm not alone in these complaints either or I'll feel like a right sad old whiner, lol.
Even with the complaints aside though, I think you've made the first true Mortal Kombat beat 'em up experience. There's other OpenBOR MK games out there but this is the first to include the whole roster (quite faithfully, too) and follow the story of the games

For that reason, it will likely be the MK beat 'em up I keep coming back to.