I have used Three since November 2019 downloading up to 300GB some months but never tested the fabled 1TB limit. Never had a whisper of complaint from Three. I bought a Huawei B525 router from Amazon reseller Livewire and got an unlimited sim only contract from Three rather than signing up to their data service with their supplied router because this way I get unlimited calls as well. The router has an RG11 socket that with an adapter enables a standard DECT wireless phone to be used. I think it's only Huawei routers that have the phone socket, and it's not all of them either. Download speeds vary significantly, 60-80mb/s early mornings and late evenings down to 40mb/s or so at peak times. I don't use Netflix but I regularly stream through my TV from BBC iPlayer and don't get any buffer problems, even with their UHD streams.
Geographic location is everything however, and you may get better or a lot worse.
There have been a couple of service faults but mobile systems have a fair amount of failsafes that a landline doesn't have, if the 4G service drops the router reverts to 3G which is what happened during the fault. If I had lost the nearest tower there's another that will just about get a signal, albeit not very fast. If the router ever broke I can take the sim out and put it in the mobile phone and use hot spot.
If your phone is newish, i.e. cat6 or better, you might not even have to bother with a router at all.
For Three's unlimited 12 month sim-only I pay £18 per month.
BTW, "LTE Inspecteur" is an invaluable program for PC to position the router for best signal, and you can also play around with band selection for best speed.