Plenty going on ... But ...
NYE isn't like most of the world. Midnight out and about is boring. Most people are with family or fixed restaurant dinners. 2 am? Clubs are packed.
It is traditional summer holiday time. Those with dough that stay "local" are in Punta del Este. Those with less dough are in Mar del Plata. Still plenty to do in the city. I've spent the first week of January there since 2009, with the exception of 2021.
Weather can range from pleasant to disgustingly hot and humid with violent thunderstorms.