if you have perimeter with flamethrowers+lasers you can beat any vanilla mutant waves, but this mod does nothing, laser still protect you from ceramics and flamethrowers protect you from mirrors. Nothing changed. Enemies should have both resistance types (fire and laser) to force player build some physical based defence, like vanilla machinegun turrets or modded cannon turrets.

Yes, if you have flamethrower + laser turrets for defense, you will be fine. But not everyone build flamethrower turrets for their base. At least in more than 50% of my public multiplayer games, people would only use laser turrets if the spawns were not that much. To my experience, players of "Death World" would see flamethrower turrets as essential (probably you are part of this group.) Players of "normal modes/maps" tend not to use flamethrower turrets. This mod primarily serves to "upgrade" the experience of the later group, by "pressuring" them to build a mixed defense.

Although you already have a good mixed defense, I am sure that at least those invasions would have pushed further into the area, compared to if you don't have those mutants. For example, a leading ceramic biter would have attracted your laser turrets to keep shooting on them, so they bought some time for the trailing spitters to attack your defense. So I am sure this mod at least "forced" you to have a denser defense line and consume more ammo to achieve "perfect" defense.

Assuming players not having other modded turrets, vanilla gun turrets has a range of 18, while behemoth spitter has a range of 16. This can be too restrictive on how gun turrets should be deployed, so I am hesitant if mutants can be resistant to both fire and laser.

another question why this mod doesn't support your other mod what adds new tiers of enemies? i checked resistances of type V and type VI and there is nothing too impressive, just more health.

I was considering that too. But for category V onwards already have slowly buildup of major resistances. For example, laser resistance of Category V biter is 10/15%, Category VI biter is 20/30%, ..., Category X biter is 60/90%. Making turrets less and less effective already.
So laser turrets become weaker and weaker, and fire turrets has inherently slow in damaging such high health enemies. At the same time, spitters have an extra attack range each tier. Category V spitter is range 17, Category X spitter is range 22.
If I allow the mixing, a Category VII ceramic spitter is immune to laser turrets, outranging gun turrets, and only slowly damaged by the flamethrower turrets. I guess this will not be a fun situation.
Plus, this is not asking players to build a balanced defense, but telling them to abandon the "useless" vanilla turrets, and forcing them to go installing some OP turrets from some mods, another thing that is AGAINST the spirit of my modding, something I want to avoid.
Yeah, so this is another reason not to have BOTH fire and laser immunity on the same mutant. Because the vanilla three turrets (gun/laser/flamethrower) would be all useless against such high tier spitter.