the people that were here before us were kinda wimpy. Their homes are all starter homes with just enough space for like a bed and a chest. Their defenses, stick fence mostly, wouldn't stop a hungry boar. They all got killed. Their skeletons wield at best crude bows and bronze swords. I can't see even a whole village of them taking out one fuling.

rotting, brainless and bannished i dont believe draugr and undead can mess with a fuling horde. the draugr where once strong warriors but scince they got banned. they are just rotting to be only dangerous in larger groups

I guess you never watched the movie Gremlins. Rotting corpses with pieces falling off don't seem to be very tough to me, especially the ones that have had all their flesh drop off and are just animated bones. Leeches are weak sauce. Blobs are simple and even oozers are nothing to worry about (even in Bronze gear) if you have poison resist meads. An Abomination is no match for a Fuling Berserker (especially if backed up by a shaman) and I could tell that the very first time I went up against a Berserker. The most difficult thing about the swamp is navigating it.

There'll be more dead stuff in the future biomes, apparently.

If the ancient dead are so strong, why'd they die in the first place?

Zombies: 0
Goblins: 1

I mean, rotten corpse vs rather lean and fully intact goblins that sometimes have the hulk or a shaman to back them up. I know which one i find more concerning (unless im on my boat, those undead apparently entirely ignore water physics or suddenly develop hulk strength to shoot arrows through several feet of water with no momentum loss...I get the lack of needing to breathe, dont get that physics only sometimes applies to them). That...and deathsquitos exist but by tier 3 your best defense against them is the serpent shield and/or fast reflexes. Although admittedly, normal mosquitos do usually prefer standing water so i always thought those actually suited swamps better if not for their one shot glass canon setup. Lox does seem fitting for plains though and those will also wreck your tier 3 day rather quickly.