I'm never happy when I see 'besides' in a Task 2 essay.
My advice to the Members Academy is always DON'T USE 'BESIDES' as a linking word.
What's wrong with 'besides'? I can never quite explain why it's wrong but I think the meaning is not what it looks like. It looks like a synonym for 'also' but it doesn't work like that.
Sheldon puts it more simply - he says it's wrong because it's INFORMAL.
So it's wrong grammatically/in meaning (in my opinion) and also it's not formal enough for Task 2.
Here are some examples that show how 'besides' usually means a mixture of 'apart from' or 'as well as' and (in some cases) 'and anyway' rather than "also':