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Well, I'll share my opinion too. It's not just Magistar who should write extensively:D
So, I'll talk about the preference for choosing an upgrade or an alternate upgrade.
1) 70/30 As mentioned Magistar said, sorry for the tautology, the destructive spell of the Cave Lords can be useful, and with the help of Imps, you can cast it endlessly. But I think that Imps are much more useful because you can consider that the school of darkness is a priority in the Inferno castles, and it will not be difficult for a Demon to develop his dark magic, and it is unlikely that a unit's spell will surpass the spell of its lord in power), therefore Imps are more useful. And a full arsenal of dark magic clearly defeats the Fireball and Meteor Shower. Of course, if you have long surpassed a hundred Cave Lords, then their magic is stronger, but in this case, it's strange that your game has dragged on to such an extent.
2) 60/40 Yes, yes, you're not mistaken, and you don't have any glitches. My train of thought is this: let's not take stats into account, the differences are insignificant, except for attack and defense. If we play with an alternate upgrade, then jumping over half the field and dealing increased damage is good, but the downside is the increased damage of the counterattack, and besides, the alternate upgrade (it's like the name of the upgrade and alternate upgrade was mixed up:D) has lower defense, and after jumping an Olympic record, it will remain for stretching, because it won't retreat, and it's a pity to spend a hero's turn on teleporting to move a level 2 unit, all this makes the alternate upgrade less long-lived, and after all, Demons are the main meat of Inferno (not Imps!). And now imagine that you have a surprise attack (it is useless for the alternate upgrade, and it is not so difficult to develop this skill, and if you play as Grok, it's even better), then you can throw upgraded Demons into the enemy's ranks, increasing their initiative, and if you wisely developed the Demon before and pumped up its initiative, then the upgraded Demon will go almost immediately after the hero, and with an explosion, the power of which depends on the number of creatures, it will deal a painful blow, and also block the shooters.
3) 20/80 Here, the Hellhounds clearly win. Even despite the lower attack, hitting three, and then another three with Hellfire, and all for free, and if the hero's ability Hellfire works, then there's also a bonus to the damage, which is very tempting. The third level is not made for defense, so the higher defense of Cerberuses is not impressive.
4) 60/40 As in the case of Hellhounds, the Hellfire also applies to Demonesses, and the chain of shots is very stressful, and physically it is also noticeable, as for Succubi, yes, it's cool to control troops, but don't forget, we have a full arsenal of dark magic, which means we can cast the same control with the hero, and the only shooter can be used more radically when the enemy has two types of shooters.
5) 50/50 Seriously. I can't decide on the choice, since relying only on stats is not entirely correct, you need to take into account the potential of the units, and Nightmares and Hell Steeds have almost the same potential, the difference is that one scares nearby, and the other hits, both at the beginning of the turn. And honestly, I have never observed the effect of these creatures' auras. Therefore, here it is unclear, again, I don't want to rely on stats, there are many factors that go against the stats, and there are fewer factors that go against abilities.
6) 70/30 I don't think I need to explain why the only casters should not be removed from the squad. I will only say that it is more important to remove the extra lives of high-level creatures for the 6th level of Inferno, since they can be bred by cloning and summoning, well, and with the Archdevil, a bunch of Cave Lords, who will not give the enemy peace. But it is important! It makes sense to take Cave Goblins when fighting against the Academy and Necropolis. And so, to destroy numerous low-level creatures, there are Demons, Hellhounds, and partially Demonesses.
7) 80/20 I think Archdevils are better, because capturing an Archdemon is extremely doubtful, and there are few cases when it will be useful, especially since after activating the ability, it ends its turn, which is not very good for a pumped-up level 7 creature. But Archdevils, especially if you play against Necropolis, are good, not counting the superiority in speed compared to the Archdemon, the destruction of corpses prevents them from being resurrected, a kind of replacement for the hero's Corpse Consumption, which is more useful than a hook.
Phew, that's what I said. I drew conclusions based on personal experience, and a little on other people's opinions and strategies, don't be offended if my ratios upset you.