"You are likely to gain weight."
It is fairly trivial to stock your cabinets and refrigerator at home with healthy food options. Contrast this with an office environment. Between the morning donuts, the snack bins, the beer kegs, the pizza delivered for meetings, and the endless birthday cakes - sweet lord, the birthday cakes! - it's far more likely that you'll gain weight there than at home.
I also switched to a standing desk about six months back so I don't sit hunched over in a chair all day long. This would be difficult to do in a cube or open planned office (the environments I worked in before I started my consultancy).
I also get a lot more sun than I'd otherwise do because I can pick the nicest spot in the house to work rather than being stuck in some big room surrounded by fabric walls. Plus I can open the windows in the summer and get plenty of fresh air --- and walk around without disturbing people.
All in all, I find my home work environment much healthier than any office I've ever worked in. The only downside is that when I had to commute, I'd commute by bicycle as much as possible, so now I bike much less than I used to.
I'd say switching from a bicycle commute and a sedentary 8 hours in an unhealthy office to no commute and low-intensity exercise at a standing desk in a healthy environment has been a wash for me. But the bicycle was the only healthy part of my old situation, so for people who don't do that, I can't see any downside to working at home unless they make really bad choices.
In a WFH situation, it's actually the lack of exercise or commute that I'm talking about.
Walking or biking to work and walking with co-workers to and from lunch several times a week is healthy.
One has to make up for these lost opportunities by exercising more simply for the sake of exercising.
The social element of work has a lot of advantages.
WFH took me from riding/walking 20 miles per week to near zero.
It's taken a lot of discipline and time to duplicate that.
I'm trying to compensate by going for a longer walk after-office, but because it's not a necessity I'll drop that far more often than not. I'm a lazy bastard, after all ...