Stage 1 vs follow on milk

[elephantfeels]

So when DS was born we were told not to go near any other formula/milk other than stage 1. We were told it's just money making and there is no difference. DS is now one but still having one bottle of formula a day due to his weight. I couldn't get any stage 1 today so got some follow on milk to tie us over should we need too.
Anyway upon looking it is clear that there are added vitamins etc in follow on milk so in theory it does offer baby more in addition although it might be 'money making' its cheaper than stage 1 milk and means you are able to get offers!
So what is so bad about follow on milk? Can anyone explain why HV etc say not to touch it?

It was invented to get round the rules on advertising first milk (not allowed). It is often cheaper and you can get special offers on it so I did switch to it at around 9 months. I mean follow on BTW not toddler milk. Switched to cows milk at age 1 though.

It’s usually cheaper

After 1yo just go on to cows milk

Sorry to clarify. He has cows milk but due to low weight he still has formula before bed in order to get more calories into him. (Recommended by GP)

I didn’t give it to my child who is Still in the 95th percentile at 18 months . He has managed to grow just fine.

But he was weaned at 4 months. Full fat cows milk from 1 year.