What I feel like a problems in AoW3?
I won't oppose thereafter any feelings nor suggestions, just trying and justifying or explaining how AoW3 currently works from a non competitive SP player's point of view, without any experience with Planetfall.
3)
Spell researching is overloaded with Empire upgrades, increasing of cast points and class units. So there's no reason to learn most of the spells.
Spell offer is not overloaded because each category of spells is represented in the spellbook (e.g. it's always possible to research one or several combat spells). It is up to the player to prioritize the research, depending on how the game is conducted on a strategic level. The same applies when choosing RG bonuses. I interpret this as follow: if you feel like urged to max empire upgrades, class units or casting points, then those skills have to be important, for sure, and you are also certainly prioritizing them over tactical and strategic spells. But it's your choice, and it's certainly possible, if challenging, to postpone the production of a class unit a few turns to get a class spell before.
4)
Cities require more precise production management.
Can't Haste Production solve this problem? As for other production management, I find myself that I often neglect the potential of Build Housings, Research Mana, even Produce Merchandises, to tune this management more finely. E.g. stopping production in several cities to enable one critical production in one city.
5)
Heroes overloaded.
One thing that is non user-friendly in AoW3 is the limited number of action icons on the top bar. The UI should be revised as it is not handy to open the unit panel to access the 12th ability... At least, allow the player to choose which ability can have its icon (kind of quick slots).
6)
Exp. from touching abilities.
I try to maximise experience when the fight is not very risky. But I don't understand why not every "combat summon" or any unit created during a fight wouldn't grant experience like any normal unit. Sure, this would increase the total experience you might gain in combat, but this would be only natural, imho. Currently, units that are resurgent, animated, ... don't all provide any experience, though their opponent fight (and train) for real!
8)
Killing of Heroes.
Autocombat may be in fault here, but killing and looting heroes is a valid tactic. Except for very tanky heroes, they shouldn't try and oppose mighty units one-to-one. Heroes shine when leading an army that suits them. I would be against saving their equipment, or at least, as in Warlords Battlecry, they could possibly retrieve equipped (slot) items only, and their backpack (utility items and non equipped items) could be looted, and in this case, even for leaders who are sent into the Void.
9)
Artefact craft. Useless.
I never use the forge but others do. Perhaps this is mandatory when using auto-combat?
10)
Unit enchantments.
At first, I was upset by the differences with AoW2 (also the single combat spell per round). I've came to live with them, and I could now justify the shift by arguing that you could use Unit enchantments like a fire-and-forget spell, back to AoW2, only as far as your mana income would permit it. In AoW3, provided you invested in those casting skills, you can unleash a great deal of magic dependent on the stored amount of mana, not the income (for the same effect). This is not inferior or superior, this is different. As for unit enchantements in AoW3, I guess that everybody agrees on their tactical usefulness, as they basically grant any affected unit one or several rank or even tier. The fact that the player has to choose between several spells on each round makes for another subtle level of tactics.
Another aspect of AoW3 that helps living with the new system are the empire upgrades (either class- or avatar-related); they sometimes are like "unit enchantements" for a whole category of units.
This can solve the problem with useless mana in early game.
I never have enough mana in early game: always a scout unit to summon, then to sustain. Even as a Warlord, I can always cast a combat spell whenever I need it.