Witcher 3 had a few great side quests but most were the standard kinda boring type.
Horizon had a lot of average side quests too but all the ones that involved making friends were pretty good imo. They worked because you wanted to learn more not just play more.
I'll have to completely disagree there. Here are just sample of side quests that were very well done, awesome and had an effect and the story or characters.
-Yen's side quests with the gene
-Triss side quests with helping mages in novigrad
- the tower in the middle of a skellige island that appeared out of nowhere
- helping crachs sons with the ice giant(a whole island was dedicated for this side quests for the most part)
- helping crachs daughter with the leach type monster
- both the crach children side quests were part of a bigger side quests which determines the fate of skellige
- the king radiovide assassination side quests and story line
- the side quests where you help the master sword Smith's
- the side quests where you help the master armoror
- the side quests were you help Kira and the haunted tower
- the side quest where you help the man who lost his wife and find out it was a werewolf (in velen)
- the side quest of the cursed warewolf in skellige
- the free dlc side quests of skelliges most wanted
- the other free dlc mission with the pigs
- dandelions side quests where his gf is attacked
- lambarts side quests in novigrad
- Roche side quest
- the side quests of the slaughtered village (free dlc as well)
And there are a lot more. Yes, there were more smaller side quests but they were more polished than horizons and flowed better. I'd agrue that the Witcher 3's side quests also gave a lot of info and did a lot of great world building. They gave a lot of insight on the world and what's going on, the war, the religions, the people, poverty, way of life, society and so on. And they did a much better job at it than horizon.