Here's the thing: flexibility only works if it includes the ability to say NO. In a sense, I'm always "at work." I'm a bard. I can't turn my brain off. All day long, I observe things, ideas mill around in my brain, sometimes I sit and write them out. I often feel like I should be working. In order for something to really captivate me, its value has to exceed the pleasure and productivity of writing (or gardening, or doing some other important useful task), and that's not very common. What makes it work is that I can, in fact, say no at any time. I can go to be whenever I decide I'm tired enough. I can get up when I feel stiff. I can go outside if it's a gorgeous day. If I get sick for a week, I can take that week off. My schedule will be bitched to hell as a result, but nobody can force me to work when I can't sit up. Which means that I'm working as much as I want to, and since I love writing, that adds up to a lot. Then I can, for instance, go to a fair on whichever day the temperature is low enough I won't melt five minutes after leaving the car. So that works.
Any "flexible" work situation that slops work into private time, but doesn't allow people to carve the same amount of time out of the regular workday, is going to be a disaster. People need privacy. They need time off. If you don't provide it, they'll die on you. Consider, for example, the 20-year life expectancy gap between rich and poor. It's not just about "better" health care. It's that rich people get to sleep, take vacations, and turn off their phone if they feel like it. Poor people are always on call for someone. And it kills them.