Ron Amadeo at Ars Technica has a great piece on 5G and the challenges for roll out:

[T]he move to 5G mmWave is not a slam-dunk argument. Since mmWave runs at a significantly higher frequency than LTE, that means it comes with no shortage of tradeoffs. MmWave has worse range and worse penetration compared to LTE. A mmWave signal can be blocked by buildings, trees, and even your hand. MmWave doesn’t work well in the rain or fog, and the ~60GHz chunk of this spectrum can actually be absorbed by oxygen. That’s right—a slice of mmWave spectrum can be blocked by the air.
With so many issues to overcome, mmWave sounds like a terrible chunk of spectrum to build a mobile network in until you consider two key points: the higher-frequency means mmWave has plenty of bandwidth and low latency if you can get it, and most of all, the spectrum is available. MmWave isn’t being used for much right now because it is such a pain in the butt to work with. So if you can figure out all the implementation problems, you suddenly have a vast amount of airspace to work with. That’s actually the first thing these companies talk about when they bring up mmWave. It’s all going to be really, really hard and complicated, they say, but it’s going to be worth it.
Don’t buy a 5G smartphone—at least, not for a while
If you are an intellectual property attorney in the digital space, it’s time to read up on 5G. It’s going to be an IP licensing minefield.