One consideration is that most rental car companies won't permit you to take the car into Guatemala. Crystal is one, Thrift I think still does allow it. Another is insurance -- last time I checked you couldn't buy insurance at the Melchor/Benque border, and of course Belize insurance isn't valid in Belize. Easy and safe? Well, it's a matter of how you see things. Guatemala isn't Michigan or Georgia, but it's certainly a lot safer than, say, South Africa.
If you are an American citizen you do not need an advance visa to enter Guatemala. Departing Belize you now have to pay a US$10 fee. There is also a Guatemala fee, which is somewhat more flexible depending on what the Guat officials think you will pay.