I know you said "generally", but there are some significant exceptions (like VXX): https://finance.yahoo.com/chart/VXX#eyJpbnRlcnZhbCI6IndlZWsi...
Even the prospectus says "The long term expected value of your ETNs is zero": http://www.ipathetn.com/US/16/en/details.app?instrumentId=25...