Acetaldehyde smells sour and metalic. Ethyl acetate smells like nail polish remover. Both are wine faults. And acetaldehyde is a carcinogen. It occurs naturally in ripe fruit and will be in all wines, but you want to limit this chemical for sure.
A banana smell is usually from isoamyl acetate. A bit of isoamyl acetate can add to the bouquet complexity of a white wine, but not so much appreciated * in a red wine (except fruit forward wines like Beaujolais). Typically, isoamyl acetate will decrease with age. I once have had a Pinot Gris with a slight banana bouquet its first year that was completely gone by year two.
* But I am not wine convention pretentious, and I think so called "standards" should be constantly questioned and re-evaluated.