Some fat can remain lodged in the liver and contribute to fatty liver disease. High intake of fructose is also associated with obesity, metabolic syndrome, insulin resistance, and type 2 diabetes.   
HFCS 90 is likely more problematic due to the higher fructose content (90%). Overall, high fructose corn syrup and table sugar seem to have similar negative health effects because they both contain about half glucose and half fructose. Research shows no difference in feelings of fullness, leptin levels, effects on body weight, and insulin response.  
People are also concerned about the use of genetically modified corn syrup and the long-term consequences of consuming genetically modified foods. More research is important to assess the risks of GMOs.
Just because excess fructose is a bad idea for your health doesn’t mean you should avoid fruit completely. Fructose in the natural form from fruit comes along with nutrients, antioxidants, fiber, and other advantageous compounds. It’s difficult to overeat fructose if you’re only obtaining it from whole fruit. Keto dieters limit fruit to remain in ketosis and stick to keto-friendly fruits like berries. 
Concluding Thoughts on HFCS
HFCS is a type of sugary syrup made from corn. Keto dieters typically avoid sweeteners like high-fructose corn syrup to remain in ketosis! If you’re keto, you won’t be eating HFCS anyways, and you’ll be choosing keto-approved sweeteners like stevia, monk fruit, and cinnamon!
The negative health effects of HFCS seem to be worse with HFCS 90, which has a higher fructose content of 90%. There isn’t sufficient evidence to suggest HFCS 55 is any worse than regular table sugar. HFCS 55 is also the most similar to table sugar and the most commonly used.