The main benefit of 5G isn't that it's really fast, but that it has very fast latency (typically 1ms). This will be a boon for gamers, and generally things will feel like a desktop on a wired fiber connection. 4G is plenty fast as it is, but latency is variable.
5G is licensed to operate at 3.5 GHz (microwave) and 24GHz+ (millimeter wave) bands. As a consequence of this new spectrum allocation, cell operators will have to install new towers, but at much greater density than before since mm wave is hardly penetrating.
One problem with 5G (and even existing cell tech) is that studies do show that rats showed a higher level of brain tumors and rare tumors. Other studies show effects in people, but it's in dispute. So if you have to put up a lot more towers to increase coverage, this doesn't seem like a great technology to deploy to the masses and then beta test the effects of this radiation on people.
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Apple waiting till 2020 is fine since it's rolling out in 2019 in select cities, and they can observe the issues with early tech as they did with their 4G rollout.