He takes a lot of space especially on the CT-side, which forces other players to play around him (in this case nexa and huNter-). This forces them to focus a lot more attention into supporting NiKo which makes them play worse especially if they are not so used to their new role. huNter- seems to be back in form, but nexa has struggled a lot individually ever since NiKo joined.
Also huNter- and NiKo were both basically playing all the same positions, so huNter- had to learn other positions so NiKo could take the ones he was playing on FaZe.
Edit: Forgot to add this; NiKo does make a lot of space for his team, which is good, but whenever he is playing bad or when he is getting countered hard it's quite impossible for G2 to get anything done if they mostly rely on NiKo getting all the kills. Of course if huNter- or kennyS pops off for some reason then it might be possible, but they are relying on NiKo's individual game too much imo.