He says he doesn't collect cameras but instead he sells them. My take is that the bottom tranche of cheap cameras is awful but that you have a huge selection of used cameras on Ebay, in his shoes I would have expected to get something used but good for $200 or so.
One of the reasons I go around with two Sonys in my backpack is that I can go to an event and take action shots while I put the other body with a 90mm lens and have somebody else who doesn't know a lot about how to work a mirrorless shoot headshots. On the other hand, I do collect weird cameras and you might find I have two stereo cameras in my other bag.
Do you let guests shoot the 90mm in full auto mode? Personally I've found that the general population has a really hard time operating a camera. My partner and I can both do photography though I'm more serious about it and we frequently travel with cameras. When we want someone else to take pictures, even on Auto mode I find others have a hard time. If we're in a hurry and want a picture taken by someone else we just hand them a phone.
Usually it would be aperture priority (f/2.8) in a situation where I know what the light is so I can set the ISO and leave it there. I develop with DxO so I am not worried about noise or the shutter speed too long but I do worry about hitting 1/8000 sec -- so usually it would be a situation where the lighting is predictable.
The autofocus can be set in a mode where it will reliably lock on the subject's eye. I would demo how you have to be a certain distance to get a headshot, since it is a prime lens, if they are too far away I don't worry too much because modern cameras have a lot of pixels.
For me it's the AF. Every. Single. Time. People on smartphone, used to near-infinite depth of field, just forgot/never learned about focusing, and handing them a camera just too often results with the background being in focus and the subject blurry.
Yeah that's been my experience too, or if the lens is open wide enough they don't watch the AF and have it focus on a belt or something weird and then blur out faces and eyes.
The other big one is HDR. HDR on phones makes lighting a lot less of a factor but a lot of times if I'm asking a friend to take an indoor picture they underexpose the shot because they don't have good lighting.
One of the reasons I go around with two Sonys in my backpack is that I can go to an event and take action shots while I put the other body with a 90mm lens and have somebody else who doesn't know a lot about how to work a mirrorless shoot headshots. On the other hand, I do collect weird cameras and you might find I have two stereo cameras in my other bag.