Do the AI changes alone make the enemies harder? Some of the new enemy types are way too overpowered. Literally no turret in Vanilla can counter poison biters and their instakilling stacking damage cloud.

Enemies power depends on settings you choose for New Enemies. If you set 10 tiers or even 8 tiers maximum, and allow all factions, then there is no way to survive against new enemies after evolution progress without Rampant Arsenal mod installed with new defenses (better electric/laser turrets, better flamethrower turret, MK2 gunturret or shotgun turret. Even Bob's warefare, Schall Alien tech mods don't allow to survive unless plasma cheat is heavily used.

Install Rampant Arsenal (RA) or you can turn off strong factions in mod settings for new game and via console in existing game to correctly remove. Strong factions: Poison, Nuclear, Laser and Inferno. These four need tactics approach in defense line and attack weapons + RA damage tools (Laser/Electric + Explosive damage vs Poison, Explosive dmg vs Nuclear, poison damage vs Laser and Inferno factions). Harden pipes mod is also useful to protect flamethrowers turret ammo supply. Additional electric power lines to be ready for main line destruction.
Better walls (RA has two nice types - mending wall for midgame and reinforced walls for endgame to be paired with repairing towers.

It depends on how deeply you want to dive into warfare in Factorio or prefer to focus on building big mega-factories.

it does, altho i would recomend making just the unbalanced types not spawn, or add rampant evolution, as that mod alows devolution (to a point) and alows control over how fast they can evolve by making you factory run cleaner, hunting them, and a couple other things, but in exchange polution and other effects that increase evolution are worse, as its based on tree damage and land obsorbing polution, rather than nests getting bothered