Just wondering if they had the constant damage health effect, obviously Serana is exempt as she's undead and unaffected.

In theory, Lydia for example should slowly die if you take her into it.

I've only taken Serana.

Anyone taken a follower? Or an armoured troll/husky?

Well i cured serana from her vampirisim and then i took her to the soul cairn and she was fine

I tried taking other followers but you can't take them into the tower where the portal is so...nope only Serana. Permanent summons can't follow you into the soul cairn either, unfortunately, because they have the same problem.