It's very circumstantial, so individual responses may turn out to be red herrings without a higher level analysis.

That said, I am one of these people and here are the warts in my specific case (which is not exactly what the 4 Day Week Foundation is arguing for): less money and a tendency to go out less - somehow, being forced to travel to work also made me keener to go out more.

Other than these, which are not major issues for me, things are great and I can't recommend enough to people to reduce their workweek and spend more time with their family, friends, take on hobbies, volunteer, do sport, learn new skills, get involved in their communities and civically, and generally whatever they wish to spend their brief existence doing. It's been a boon for my mental health and it's allowed me to flourish in some respects. I appreciate it won't be the same for everyone, but we should absolutely offer this option to citizens as a society.