Your smart devices will treat Pokefi WiFi a normal WiFi network, therefore you will no in data safe mode while using the Pokefi. This means the Pokefi will use more data than in cellular data network.
In particular, so-called streaming services such as YouTube will deliver in higher quality if you are on WiFi network including pocket WiFi. For example (refer to the screen captures), a 33 seconds SmartGo video will use ~5MB data using 4G, it will increase to ~11MB using WiFi. The longer the video the bigger the differences. It could be up to 32 times more in data usage for HD video using WiFi network.
There are many Data Monitor apps available you can use to check data usage by yourself.