It is quite easy to inject a javaScript code with a MITM-attack threw the public wifi into your device. So you could setup a monero miner on other peoples devices without their knowledge. Following from this I would recommend to avoid free wifis. You can search for Mining Monero with MITM attack [Bettercap] on youtube if you want to know more about it.
Miners are nasty ones, I experienced it on my browser and my laptop started heating up and everything got extremely slow. I used to laugh at the term Man In The Middle (mitm) but unfortunately it hurts real bad in the middle where you do not expect it
