I’ve never done this and would never do it, for all the reasons people have already described.

I would, however, choose a 6-hour train over a 2-hour flight, as long as I traveled in (European) first/business class with a seat reservation.

There is almost no amount of money that could convince me to travel 36 hours by bus if I could instead spend 5-6 hours going through airports and only one flight. If I literally didn’t have the money to fly, I would spend all my energy figuring out how not to go at all.