In bmups retreat is a naturally great strat. It's not impossible to deal with, but it's always worth thinking about this when removing existing conventions from bmups.
You may think "BOG ARE YOU STUPID? JUST MAKE ENEMIES MOVE A BIT FASTER THAN THE PLAYER" and yes I am but it's not that simple. Lets say you do that, now what happens when the enemy runs up to the player & jabs? First question is at what distance will the enemy start the jab? They probably won't initiate the jab right on top of the player, they will probably do it from a slight distance, meaning the entire hitbox won't be a factor, just a part of it. Even moving the enemy closer won't necessarily help cause players can start dodging into enemies.
Ok that's a bit of a complication but no big deal right? Wrong! Cause think about what happens when you add startup frames to this equation. Even fast unreactable attacks of 10 frames will require for the player to just move slightly out of range. The hitbox size will have to be longer than the player's movement speed * startup frames to hit a retreating player.
Or the enemy will have to slide forward to attack. But then what happens? Things like the Z axis come into play, you will have to catch the player on 2 axes, otherwise the forward move you just added will be a liability. Things get more complicated in 3D, where you have the same issues along with (usually) smooth movement around enemies. Now, because enemy attacks have to be aimed in 3D space, you have attack start position, attack hitbox size/position, attack startup frames AND tracking speed to deal with. Tracking in particular further enables moving TOWARD enemies as a strat.
And that's just one aspect of the games - enemy attacks. The other problem is stuff like AI. You can program enemies to surround the player, but if the player can keep retreating then their path towards the player will more and more resemble a straight line the more players run away. And because enemies will presumably have static-ish movement speeds, a player retreating from enemies and then quickly moving back towards them will have a neater, tighter enemy formation to deal with even if enemies actively try to spread out.
Then there is the question of space - traditionally bmups have borders of the screen past which players cant advance. Remove the borders, empower retreat. 3D games make this even more complicated because of circle strafing (which jives well with tracking) enabling players to basically go in a "straight line" with no interruptions or ways for enemies to easily block off space. Even tightening arenas isn't a great solution because it merely reduces the radius of the circle. Tank controls are probably the best way to solve this, while being able to strafe and look in a direction independent of your movement is going to be brutal and hard to balance naturally.
But it doesn't end there - now think about player speed. Retreating is hard to shut down is the player's speed is static, but if they have a run now you have to solve the same problems at multiple speeds while also factoring in that speeds can be changed at any time, even during an attack. Let's now also add a jump - you're solving the problem vertically as well. Now add iframe dodges, try to figure out a solution to that nightmare while factoring in everything else. Then add long range running attacks that let you blast a bunch of enemies at once. Then add a lot of juicy hit properities like knocked down enemies being able to knock down other enemies.
I want to stress again that none of this is unsolveable. I just wanna emphasize how fucking powerful of a strat this is, and how much effort it takes to solve it via natural means. The solutions will also mostly be intuitive and playtest driven so there will always be a ton of gaps and imperfections.
Gamers who think that they can have their epic go fast shmoovement AND incredibly balanced aggrressive enemy AI which feels completely fair have no idea what they are asking for. Fuck you gamers. Along every step, devs are fighting for their life, trying to shut down retreating players.