I've heard this a lot, but it sounds a bit too convenient to me. When external (or internal) circumstances have forced me to spend lots of time on one specific, not particularly entertaining task, I've found that I actually become more interested and enthusiastic about that thing. For example, when I had to play chess for like 5 hours a day for a week once, or when I went on holiday and came back to 5000 anki reviews, or when I was on a maths camp that started every day with a problem set that took over 4 hours.

Re "breaking down": if you mean they'll have a breakdown of will and be unable to continue working, that's an easy problem to solve - just hire someone to watch you and whip you whenever your productivity declines. And/Or chew nicotine gum when at your most productive. Or something. If you mean some other kind of breakdown, that does sound like something to be cautious of, but I think the correct response isn't to surrender eighty percent of your productivity, but to increase the amount of discomfort you can endure, maybe through some sort of hormesis training.