Pay attention to the details. Most "unlimited" plans are for a particular technology delivered by a particular provider, and the rules change when your data comes through a fallback tech or you are roaming. Thus they might work well for a heavy user of data at an appropriate location but can be a problem when you are moving around, particularly when roaming.
I buy more data than I use (4GB for five devices) on a plan open to global roaming, and it is a lot cheaper for me than unlimited local offerings. When I go International I buy 250 MB plans one at a time rather than paying the international roaming fee.