Ok, wall of text time.
It's deep and diverse as far as player options go, but as soon as it comes to AI, it's not even on par with Civ V, sadly.
I remember Civ V AI being mentally challenged and allowing me to take its siege units with infantry every single time. Either you're exaggerating or Firaxis drastically improved the AI in the expansions. I wouldn't know because I uninstalled after realizing CiV had killed the Civilization franchise.
As for player options, you do realize that,
compared to EL, the faction system of Civ is merely just a bunch of reskins and different colors, right?
EDIT: Also, Civ 4 had much better use of land, allowing to drop city pretty much anywhere.
Not "better". Civ 4 adopted a different approach to the "borders question" (culture mechanic) while keeping the old single tile city system + 2 tile exploitation radius. Anyone who ever used Google Earth knows that (single tile + 2 radius) is completely unrealistic. Civ V tried to fix that but only made it worse (just like everything else those retards tried to "fix"). EL brought its own solution to the problem with the region system. You drop a city, you claim all the lands surrounding it. The city grows, you add more districts. Makes sense to me. Of course EL's system comes with its own problems, namely not being able to claim a tile that is right next to your city just because the regional border is there (despite the fact that you also own the adjacent region). They could improve on that by making the region system more flexible, allowing you to incorporate land from one region into another if you meet certain conditions.
Civ 4 allowed for more than 6 units in the army.
Terrible comparison. Civ 4 was the last game that used the old stack of doom/unit vs unit system of civ games. Since there is no army-on-army combat, the size of each army is irrelevant. You could have compared it to Call to Power or GalCiv. CTP had a 9 units per army limit while Galatic Civilizations uses a logistics system. Pick your poison: army size or unit-on-unit combat.
Civ 4 had more tech variety.
Agreed. The tech system in EL feels shallow and I blame the lack of prerequisite techs. Being able to research Titanium/Glassteel weapons 2 w/o researching Titanium/Glassteel 1 is pretty stupid.
EL is more gamey and 'balanced'
Yes. And the fact that EL took the hard way out regarding balance (
huge differences between factions) and still managed to succeed proves that the people behind it actually can into math and have the talent necessary to put a game of this scope together. Unlike present day Firaxis.
but not really all that deep.
Some mechanics are deeper, some are shallower. See above. Unless you count AI as "depth", in which case the AI is indeed pretty bad. Probably on par with Civ IV without any expansions, if you can remember that.
Also, in multiplayer the AI is not very relevant and EL has a pretty active MP scene, tho I haven't joined it myself yet.
To wrap it up: I see EL in the same light that I see Cities Skylines. The Civilization Throne is empty, just like the Simcity "throne". EL doesn't quiiiite "replace" Civ's corpse, but it's the best thing we got so far. Same with CS/Simcity. Unless you enjoy Civ V/VI and Simcity (2013), in which case you should kill yourself.
Civ IV AI could pull off some pretty impressive doomstack blitz and seize my capital a few games where I neglected armies entirely to focus on tech. Not sure if it was mods or vanilla but seems like a even vaguely threatening combat AI is impossible with 1UPT.
Pretty sure it was a mod. Civ IV AI is unimpressive, even with all the bling blings attached to it. Here:
https://forums.civfanatics.com/threads/bts-better-bts-ai-download.297810/
Better BTS AI makes the AI actually play to win. There are other AI mods, but they are mostly branches of BBAI. There is also K-Mod:
https://forums.civfanatics.com/threads/k-mod-far-beyond-the-sword.407049/
This one makes BBAI work with BUG. It also brings some balance changes and performance improvements too. This mod influenced several other mods like Realism Invictus. Karadoc is, sadly, long gone but K-Mod was finished when he left.