I would not let someone I didn't know borrow one of my machines if I wasn't there. If I was there in a class setting, I would have no problems letting someone sew on any of my machine.
My machines are all vintage except for my cheap Brother that is just for taking to classes. The vintage machines are sturdy and I got them for cheap. I've lent them to friends for extended periods of time when someone's primary machine is in the shop. I make sure they are threaded correctly and set up for the appropriate use (quilting or zigzagging for example), along with basic supplies in a kit.
A friend of mine is giving me her old Bernina, even used it is worth more than all my machines put together and more than my computer, but less than my car. That machine will not leave my sight/house ever.