Okay, some good news regarding Fungal Caverns.
First of all, the Yog-Sothum enemy, the one that shoots small homing bugs, has been reworked in a previous update. It still now has a unique projectile, as opposed to reusing the Exotech Larva's bugs. It's much slower and there's a delay before its explosion, where it makes a distinct noise and visibly swells up before exploding. The Myconid's running speed has also been reduced significantly. (I'm also thinking of giving it a slow turn speed.) Second, most of the problematic fights have already been reworked. The gnaar brutes in the "bad spider fight" have been replaced with small lava golems. In the big hall with the masks, the sheer amount of enemies has been greatly reduced, along with additional health and armour pickups added. The dark fights' waves have been shortened as well. Though the last one I'm still going to rework a bit, since I did rush its design a little. Maybe add some additional tunnels the enemies run out of, as opposed to spawning right on the ground. Also, a minor thing, but the glowing mushrooms now have static light, instead of the animated pulsing. This was originally because textures were bugging out from all the lighting updates, but it did help in the dark sections a little bit.
You said something about cannonballs. Your version seems to be the same that Serious Vovan played, so it might be before the update that introduced weaker cannonballs. Jade Guardians, Mammoths and Cannon Turrets all shoot weaker cannonballs now, so they don't destroy you in one hit (it might be a different story on serious).
Anyway, all of these levels are still going to receive more updates in the future. The very big update I'm currently working on, is going to include some updates on the currently existing maps.