How come lots of people are wary of melatonin?

Hey gang, life long insomniac here, never been able to sleep and it's been really bad lately.

I went to see a sleep doctor, and after doing some CBT (which didn't move the needle much), he prescribed me some Dayvigo and a compound liquid melatonin.

He said to use the Dayvigo regularly, which I have been. It sort of works, but doesn't do a lot for me. Also advised to use the melatonin on days of high stress/when I really need a sleep.

I've seen a few posts on this sub where people recommend not using melatonin, or when they stack, they exclude it. Is there a shift in how effective melatonin is for people? I'm just curious as I'm trying to find something that works for me but am seeing a fair bit of conflicting info.

Thanks all :)