From experience, let me assure you that if you think the points everyone is making are overblown or not valid, you are mistaken. Vid and Lloyd summed it up pretty good. I don't have a lot to add other than:
-Kids sitting and standing on your games. People sitting drinks on your games.
-Restaurants and locations have fires. Be prepared to lose everything.
Your games need to be gone over and need to be bullet proof. That little annoying ball hang at home that you have to pull the glass and free up becomes a major problem when the game is on location. It may happen once a week at home. It will probably happen every day on location.
Don't expect to put the games out and have money just pour in. Things will break that you had no idea could break. You'd better have parts on hand to fix it or your game will be down. Shooter rods, flipper bats, plunger links, switches, optos, springs, stops. DMDs will go dead. WPC display controller boards will fry.
If you are running anything older than about the year 2000, you'd better have spare boards on hand. SAM boards are pretty reliable and are new enough that they don't have tons of issues. Once you start getting into stuff older than that, it usually has been played quite a bit and stuff breaks. You'd better have the parts on hand to fix it. And your WPC game that has never reset once in its life, well...don't expect that to continue once it has been on all day every day. You'd better know how to properly fix it, reseating the connectors and replacing BR2 and C5 isn't going to cut it.
I can go on, I'll stop there.