I've been with 3 for 4 or 5 years now.
I understand that the unlimited data plan is for a phone, not a tablet/dongle or
mifi.
3 call it "all you can eat plan"
I've got this plan for the phone.
I've also got a 3 data-only sim that I use in a usb dongle plugged into a netgear r7000 router, this gives me 100gb pm.
I've used them both extensively around Europe and am pleased that the performance is good.
Both plans, when outside the UK limit my data to 20gb pm each.
I have previously used the phone sim with unlimited data with a Samsung Galaxy Phablet (10" 4g tablet) which is detected by the network which issues a warning that the device is not allowed with this sim plan, which then cuts off internet.
I can also plug the phone into the laptop which allows usb tethering giving internet access to the laptop through the phone.
james