>BFG reacts only to Normal Monsters, but not to Custom Ones.
Ah, you're right. It's a bit of an oversight now that I think of it. Technically, anything with custom monsters kind of needs a compatibility patch. I can probably make a Scythe 2 patch that will work, give me a little bit and I'll see what I can do.
The reason for this issue is because the BFG scans for "BabelMonster", not just "Actor" with monster tags, in order to speed up calculation and improve performance a bit, while also avoiding issues like accidentally hitting shootable actors.
>I had moments during the passage when there were Cacodemons and Cyberdemons around. I aim at the Cyberdemon, and the Big Ball flies away at the Cacodemon. Or at the End of E4 "Ultimate Doom In Name Only" BFG could not fix the target in any way to kill Spider Masterminds! Does it depend on the close distance to the enemy?
Yes, to some extent. It tries to weight their health and distance to avoid the reverse situation where you would end up shooting some big target a mile away but not the thing in front of you. I don't remember having these issues when I did those maps, but I'll go through them again, maybe something strange is going on with the new aim logic. As of right now, in order to fire, you need to be able to "see" a monster (According to GZDoom) and also fire a straight ray from your player's center to the monster's center (the general firing line) before the target is valid. There's a bit of strange logic on that second one to get around being able to shoot through other monsters, so it's possible something weird happened there.
Anyhow, gimme a few hours and I'll test all this out.