Now overused term for 'so the bottom line is...'
Comes from an investor/accounting concept when valuing a company; based on the sum of sales/what you own (net assets) vs sum of debts (net liabilities). Weighing up net assets vs net liabilities (net-net) then says whether you have an overall positive or negative business. The term is very reductive - you don't need to explain what individual parts make up the positives or negatives, or how big they are... just that the final result is a win/loss or a yes/no.
Somehow the term has gradually crept from accounting into general business jargon (net-net this deal is good) and eventually just to give an opinion where you don't feel like providing any of the detail of how that view was formed or allow for argument from another person (I've done all the weighing up and we are going this way).
I don't care if you think there are downsides. Net-net, we'll be ahead if we buy the new TV.