I don't know exactly how hard Nintendo work their employees (Japanese companies are notorious for overwork) but I know the director of the smash bros series, Masahiro Sakurai, confessed that he didn't take a day off in 13 months when designing Super Smash bros Melee back in the day. I actually think a lot of this overwork problem could be circumvented by knowing what to include and leave out of your game projects.
Melee wouldn't have been any worse for it and Sakurai could have took days off if they removed these features:
The game has 6 clone characters (Dr Mario, Falco, Ganonndorf, Young Link, Roy and Pichu), out of those 6 they didn't need to make Roy, Dr Mario or young link (maybe pichu too but Pokemon was still a big deal), the game would have already been varied without them. The clone characters take significantly less time to make then the originals but it would have still be days, if not weeks saved on development.
Trophies, although a nice feature, the game didn't need those 290 collectable trophies. Trophies don't add anything to the core game and are just there for completion and fan service. This would have saved them time on making stages too (no pokefloats).
Sakurai's ridiculous work hours on the game were self-inflicted in a sense as he was the games director and likes to take full ownership of his projects. I just he didn't drag down his co-workers along with him.
I think over-ambition is the biggest cause of overwork in the industry. Publishers and/or producers/directors are getting too ambitious with what they want their games to be and are as a result are making their game design unnecessarily slack from putting too many features into the project. Open-world games are especially terrible for this because of they need to fill their worlds and are probably the most slack game genre out there. The value of tight design is becoming really overlooked outside of indies.
https://www.eventhubs.com/news/2014/nov ... mash-bros/