I am not sure if being a founder means that you will be unhappy. Personally I feel that that by being a founder you are constantly exposing yourself to circumstances that could cause you to be "unhappy" -- there is no end to the ups and downs.

I'll attest to the article's accuracy, but that's one data point. The highs from big wins are great, but the lows hit hard and often. Celebrating (even unreasonably so) small wins is particularly important to try to keep things in balance.

I have no idea how it averages out compared to non-founders and non-startup-employees, but it's a nasty roller-coaster to be sure.