TLDR: I consider cultures very important as these alter your gameplay and I find Reaver's gameplay technically limiting, and design choices that don't feel too great. I find the Avian form useless for me. This DLC, to me, is entirely carried by it's tomes and the fact that it adds in new enemies which help mix up the variety of enemies you fight. If I could give a neutral rating, I would.
The Long:
I bought Season pass 1, out of the four DLC, Empires and Ashes is the only one, personally, that I will be not recommending. For additional context, I only play single player, usually with maximum players and largest map distance. I do not pick what is the strongest, I pick and build characters based on roleplaying for said character.
Culture: Reaver: This culture will not start with a whispering stone, which means that to gain a free city as an ally, you must conquer them. Unless you play with no free cities, declaring war on them early will really slow down your tempo and hurts your early game. If you ignore them early, you run the risk of them being vassalized by another empire, or their city crowding your space to build and the fact that they will have higher defenses which will cost you more gold for siege projects in the mid game, but because of certain reasons, you will likely already have a hard time with your economy early game, so more pressure mid is undesirable. You are also encouraged to make war with them to acquire an additional currency unique to Reavers called War Spoils (acquired by defeating free city or empire units) which you must use to recruit your T3 Cannons. This currency can also be used in conjunction with your support to recruit enemies post battle that your T2 support has used the skill Subdue on. Most factions start with 3 T1 units that are recruitable at the beginning, generally being 1 melee, 1 range, 1 scout. The Reavers have 1 melee, 1 skirmisher, 1 scout. Unlike other factions, the scout has no ranged attack. The Skirmisher, although it can immobilize with it's ranged attack, can only be used every other turn. And the melee polearm has the disadvantage of having High Maintenance which makes them cost 2 additional gold per turn. As mentioned, early game as Reavers sucks, there's no other way to put it. I don't understand the point of Reavers not being allowed to start with a whispering stone; but I also don't understand why the High culture starts with +10 good points and Dark with +10 evil. For a game that touts it's freedom and customization, it should be up to me to decide if Dark is good or evil or if Reavers are warmongerers or not. They do have the benefit to declare war and not lose imperium if the war is unjustified; why the Reavers have this but the Chosen Destroyers society trait does not, I will never know.
I have little to say about Avians. I have never thought of what can I make with a bird form. This isn't some monster musume, vanillaware harpy, this is a bird that walks and moves like a human. That said, thanks to the Emperor of Shurima, this form has been used once by me, which is 1 more use than mole or frog form in the base game.
The Seals victory point is nice to have a new way to win the game. I've played it maybe 2 or 3 times outside of the challenge realms. Because, as mentioned, I play (mostly) with max players, max world map, it is possible to win through seals without ever meeting every empire; thus it doesn't get much play from me; I prefer conquer or expansion victory.
Now, I can finally give more good points and why I kinda wish there was an it's okay recommendation.
New units, new infestation, new ancient wonders, I like these. I will never belittle more variety, unless I feel like depth has taken a nosedive as a result. Although I make fun of the frog and mole forms, I wouldn't ever say to remove them from the game. When speaking of enemies, you especially don't want to fight the same things over and over, so nothing but props here for more variety.
But, onto the tomes. The Tome of Alchemy is great, thematically, for mixing into a character that you want to be less magic focused. Mysterious Tonic is a fantastic support enchantment that gives a free action to remove 1 debuff and give 2 buffs. Antimagic Tincture is a combat spell that removes 2 debuffs and give 2 status resist in 1hex aoe. Fumigation is a pretty subpar siege project that can easily be undone by defenders assuming they have a support, and since the siege rework I don't see any reason to ever use this project. There's a building that scales a little bit if that city has a lot of magic materials. Then there is the T2 ranger Afflictor that can put Afflicting Miasma on an enemy hex. The last spell this Afflicting Miasma as a 1hex aoe. This places miasma that causes weaken, but also any unit that is damaged in the miasma, has a chance of gaining an additional debuff based on damage dealt (fire damage makes them burn, physical makes them bleed). Just be careful of your positioning.
Tome of Severing is good against magic/summon heavy enemies; with enchants to buff shield units by letting them give adjacent allies status resist. It gives enchants for melee units to disrupt enemy enchants on hit. Chance to instantkill enemy magic origin units. A spell that will banish all non-hero corpses and heal allies and give Resistance buff for each corpse removed. And a powerful T5 unit the Severing Golem which has strong ranged abilities.
Tome of the construct is all about making your race into a hive mind. Bronze Golem: T3 polearm. Compounding Defense:unit enchantment that gives any shield or polearm unit that stands next to another 1 Defense/Resistance. Then there is Linked minds is a minor transformation that makes them immune to flanks and distracting when next to another Linked mind or construct. There are two spells 1 gives hastened and strengthened, the other gives +1 retaliation attack and puts them in defense mode; both spells cascade on all units that are adjacent Linked Mind or construct and repeats.
Tome of the Dreadnaught has two AoE damage spells, one hits in 2hex aoe with chance to immobilize; the other hits in the area after a 1 turn delay for a little over double the damage of the first. A unit enchantment that buffs shields and constructs resistance. An enchantment that gives an ability to supports to overcharge a friendly construct. It heals them by 20 and makes them deal +50% damage, but has chance to stun themselves after an action. A siege project that I've never used pre or post siege rework that gives you a big cannon. And lastly, the joy of the AoW3 Dreadnaught, the Ironclad. A T4 tank with a lot of damage, that can load special ammo to stack 3x bleed, 5 burning damage and burns the tiles hit, or to ignore 50% defense and inflict 2x sundered defense.