Sugar, mainly. Oatly adds amylase to their mix which breaks down the oat starch into maltose, a type of sugar. While that is happening naturally during the digestive process since our saliva contains amylase, it doesn't happen in that amount, which means Oatly has significantly more sugar than natural oat milk. Maltose also has a glycemic index of 105, which means that it raises the blood sugar level more rapidly than even pure glucose. For reference, cane sugar has a GI of 60, lactose of 46. This means the sugar level of Oatly is effectively higher than Coca-Cola's.
If it's creamy, it's likely processed. Natural oat milk is quite watery. But just a shot in your coffee isn't worth worrying about anyway. It's just that many advertise it as a healthy, daily drink and for that, it has too much sugar.
Milk is only ever useful during your very first few years of your life after birth, after that, though, it doesn't do much. Add that to the fact that quite a bit of folks are lactose intolerant, and also the fact that I just dislike the taste of milk. That's all.