At Bearded Theory 2 weeks ago, traders were reporting problems with the WiFi and at times were unable to take card payments and so a cash backup was needed.
The issue with card payments is just keeping track of what you're spending and because you don't get a receipt (and to be fair you'd just lose them anyway) it's impossible to reconcile your statement when it lands. It might be easier if you get push notifications when your card is used, I've no experience on this.
I'll be taking cash , keeping it in the Property Lockup and just taking out my budget each day. My card will be in there too just in case.