i do not have any Bluetooth devices, so is there an advantage or disadvantage to having Bluetooth turned on with my IOS devices (iPhone X and iPad Air2)?
if you have no bluetooth devices then probably the only advantage that I can think of is having it on to be able to use airdrop. airdrop can be really handy for sending stuff between devices and even to other people when in a close proximity . it is quick and easy.
If you use your phone as a hotspot, then the Bluetooth connection allows your iPad to see your iPhone's hotspot and turn it on without you having to ever pick up your phone.