There are actually more operators than just JR and Keisei, but transferring between any of them is usually very quick and painless if you have any of the major transportation network cards in Japan, or associated NFC app. Only tourists actually buy tickets at the machines.
However, it does tend to cost more than sticking to one operator.
The fragmentation of railway companies is horrible to deal with. Tokyo is a mix between JR and Keisei and you need to buy a separate ticket for each.
At least they run really well.
There are actually more operators than just JR and Keisei, but transferring between any of them is usually very quick and painless if you have any of the major transportation network cards in Japan, or associated NFC app. Only tourists actually buy tickets at the machines.
However, it does tend to cost more than sticking to one operator.