Why are most parents against using formula ?

I’m a FTM. Initially, I wanted to breastfeed exclusively. Unfortunately it didn’t work that way because my bad had low blood sugar and sent to NICU and was given formula. I see online many argue about not using formula and makes it seem like it’s a horrible option compared to breastfeeding. Why is that ? Are there bad long term effects ? I’m using both breast milk and formula now that I can but if formula will harm my baby I’m thinking I will stop. It’s been helpful for my fiance to use while I’m sleeping/taking a break.