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When it comes to upgrading imps, a serious question often arises. Hellhounds have more balanced damage, and mana is transferred to the hero. Imps have 1 more speed, but a fourfold damage range, and mana transfer to casters, who are only pit lords, is useless in the late game.
With demons, it's simpler; I almost always do an alternate upgrade to jumpers. The explosion damage of fire demons isn't very impressive.
With hounds, there are no questions – only an alternate upgrade. With one bite, they can reach even ranged units in cover.
I almost always upgrade succubi to temptresses; seduction is just too powerful. I only use demonettes in battles against necromancers, and only if I don't have Sandro's cloak.
I'm having trouble answering about horses. Nightmares have higher initiative and an aura that reduces morale by -3. But the withering aura also deals very good damage. However, I more often use nightmares against living opponents and only use hell horses against undead.
In shorter games, I prefer pit lords divided into 2 squads. But if the game drags on, the damage from rain and a magic bolt is no longer decisive, and the initiative shifts to cave lurkers, which are excellent at shredding numerous low-level units.
The most difficult choice is with devils. Pit lords made from corpses are great. But pulling a ranged unit towards you is also very attractive. It depends on how the map is laid out